diff --git a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/optimizer/rules/ConstantFoldingRule.java b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/optimizer/rules/ConstantFoldingRule.java
index 176a678..d3093e7 100644
--- a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/optimizer/rules/ConstantFoldingRule.java
+++ b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/optimizer/rules/ConstantFoldingRule.java
@@ -43,10 +43,12 @@
 import org.apache.asterix.formats.nontagged.TypeTraitProvider;
 import org.apache.asterix.jobgen.QueryLogicalExpressionJobGen;
 import org.apache.asterix.metadata.declared.MetadataProvider;
+import org.apache.asterix.metadata.entities.Function;
 import org.apache.asterix.om.base.ADouble;
 import org.apache.asterix.om.base.IAObject;
 import org.apache.asterix.om.constants.AsterixConstantValue;
 import org.apache.asterix.om.functions.BuiltinFunctions;
+import org.apache.asterix.om.functions.IExternalFunctionInfo;
 import org.apache.asterix.om.typecomputer.impl.TypeComputeUtils;
 import org.apache.asterix.om.types.ARecordType;
 import org.apache.asterix.om.types.ATypeTag;
@@ -75,6 +77,7 @@
 import org.apache.hyracks.algebricks.core.algebra.expressions.UnnestingFunctionCallExpression;
 import org.apache.hyracks.algebricks.core.algebra.expressions.VariableReferenceExpression;
 import org.apache.hyracks.algebricks.core.algebra.functions.FunctionIdentifier;
+import org.apache.hyracks.algebricks.core.algebra.functions.IFunctionInfo;
 import org.apache.hyracks.algebricks.core.algebra.operators.logical.IOperatorSchema;
 import org.apache.hyracks.algebricks.core.algebra.visitors.ILogicalExpressionReferenceTransform;
 import org.apache.hyracks.algebricks.core.algebra.visitors.ILogicalExpressionVisitor;
@@ -356,6 +359,12 @@
         }
 
         private boolean canConstantFold(ScalarFunctionCallExpression function) throws AlgebricksException {
+            // skip external functions that are not implemented in Java
+            IFunctionInfo fi = function.getFunctionInfo();
+            if (fi instanceof IExternalFunctionInfo
+                    && !Function.FunctionLanguage.JAVA.name().equals(((IExternalFunctionInfo) fi).getLanguage())) {
+                return false;
+            }
             // skip all functions that would produce records/arrays/multisets (derived types) in their open format
             // this is because constant folding them will make them closed (currently)
             if (function.getFunctionIdentifier().equals(BuiltinFunctions.OPEN_RECORD_CONSTRUCTOR)) {
diff --git a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/AqlExpressionToPlanTranslator.java b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/AqlExpressionToPlanTranslator.java
index c0d6f82..b93335b 100644
--- a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/AqlExpressionToPlanTranslator.java
+++ b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/AqlExpressionToPlanTranslator.java
@@ -35,6 +35,7 @@
 import org.apache.asterix.lang.common.expression.VariableExpr;
 import org.apache.asterix.lang.common.statement.Query;
 import org.apache.asterix.metadata.declared.MetadataProvider;
+import org.apache.asterix.metadata.entities.Function;
 import org.apache.asterix.om.types.BuiltinType;
 import org.apache.commons.lang3.mutable.Mutable;
 import org.apache.commons.lang3.mutable.MutableObject;
@@ -77,6 +78,11 @@
     }
 
     @Override
+    protected Function.FunctionLanguage getFunctionLanguage() {
+        return Function.FunctionLanguage.AQL;
+    }
+
+    @Override
     public Pair<ILogicalOperator, LogicalVariable> visit(ForClause fc, Mutable<ILogicalOperator> tupSource)
             throws CompilationException {
         LogicalVariable v = context.newVarFromExpression(fc.getVarExpr());
diff --git a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/LangExpressionToPlanTranslator.java b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/LangExpressionToPlanTranslator.java
index 9df621c..f3480d5 100644
--- a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/LangExpressionToPlanTranslator.java
+++ b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/LangExpressionToPlanTranslator.java
@@ -72,7 +72,6 @@
 import org.apache.asterix.lang.common.util.FunctionUtil;
 import org.apache.asterix.lang.common.util.RangeMapBuilder;
 import org.apache.asterix.lang.common.visitor.base.AbstractQueryExpressionVisitor;
-import org.apache.asterix.metadata.MetadataManager;
 import org.apache.asterix.metadata.declared.DataSource;
 import org.apache.asterix.metadata.declared.DataSourceId;
 import org.apache.asterix.metadata.declared.DatasetDataSource;
@@ -166,7 +165,7 @@
  * source for the current subtree.
  */
 
-class LangExpressionToPlanTranslator
+abstract class LangExpressionToPlanTranslator
         extends AbstractQueryExpressionVisitor<Pair<ILogicalOperator, LogicalVariable>, Mutable<ILogicalOperator>>
         implements ILangExpressionToPlanTranslator {
 
@@ -885,22 +884,20 @@
         return f;
     }
 
+    protected abstract Function.FunctionLanguage getFunctionLanguage();
+
     private AbstractFunctionCallExpression lookupUserDefinedFunction(FunctionSignature signature,
             List<Mutable<ILogicalExpression>> args, SourceLocation sourceLoc) throws CompilationException {
         try {
-            if (signature.getDataverseName() == null) {
-                return null;
-            }
             Function function =
-                    MetadataManager.INSTANCE.getFunction(metadataProvider.getMetadataTxnContext(), signature);
+                    FunctionUtil.lookupUserDefinedFunctionDecl(metadataProvider.getMetadataTxnContext(), signature);
             if (function == null) {
                 return null;
             }
             IFunctionInfo finfo =
-                    function.getLanguage().isExternal()
-                            ? ExternalFunctionCompilerUtil
-                                    .getExternalFunctionInfo(metadataProvider.getMetadataTxnContext(), function)
-                            : FunctionUtil.getFunctionInfo(signature);
+                    getFunctionLanguage().equals(function.getLanguage()) ? FunctionUtil.getFunctionInfo(signature)
+                            : ExternalFunctionCompilerUtil
+                                    .getExternalFunctionInfo(metadataProvider.getMetadataTxnContext(), function);
             AbstractFunctionCallExpression f = new ScalarFunctionCallExpression(finfo, args);
             f.setSourceLocation(sourceLoc);
             return f;
diff --git a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/SqlppExpressionToPlanTranslator.java b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/SqlppExpressionToPlanTranslator.java
index 82dc344..3023305 100644
--- a/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/SqlppExpressionToPlanTranslator.java
+++ b/asterixdb/asterix-algebra/src/main/java/org/apache/asterix/translator/SqlppExpressionToPlanTranslator.java
@@ -82,6 +82,7 @@
 import org.apache.asterix.lang.sqlpp.util.SqlppVariableUtil;
 import org.apache.asterix.lang.sqlpp.visitor.base.ISqlppVisitor;
 import org.apache.asterix.metadata.declared.MetadataProvider;
+import org.apache.asterix.metadata.entities.Function;
 import org.apache.asterix.om.base.ABoolean;
 import org.apache.asterix.om.base.AInt32;
 import org.apache.asterix.om.base.AString;
@@ -161,6 +162,11 @@
     }
 
     @Override
+    protected Function.FunctionLanguage getFunctionLanguage() {
+        return Function.FunctionLanguage.SQLPP;
+    }
+
+    @Override
     public Pair<ILogicalOperator, LogicalVariable> visit(Query q, Mutable<ILogicalOperator> tupSource)
             throws CompilationException {
         Expression queryBody = q.getBody();
diff --git a/asterixdb/asterix-app/src/main/java/org/apache/asterix/app/translator/QueryTranslator.java b/asterixdb/asterix-app/src/main/java/org/apache/asterix/app/translator/QueryTranslator.java
index 93fcc7e..dcf2f2d 100644
--- a/asterixdb/asterix-app/src/main/java/org/apache/asterix/app/translator/QueryTranslator.java
+++ b/asterixdb/asterix-app/src/main/java/org/apache/asterix/app/translator/QueryTranslator.java
@@ -23,7 +23,6 @@
 import java.io.InputStream;
 import java.rmi.RemoteException;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Date;
@@ -33,12 +32,12 @@
 import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
-import java.util.stream.Collectors;
 
 import org.apache.asterix.active.ActivityState;
 import org.apache.asterix.active.EntityId;
@@ -1840,13 +1839,20 @@
                 }
             }
             if (cfs.isExternal()) {
-                Function.FunctionLanguage functionLang = Function.FunctionLanguage.findByName(cfs.getLang());
-                if (functionLang == null || !functionLang.isExternal()) {
-                    String expectedExternalLanguages = Arrays.stream(Function.FunctionLanguage.values())
-                            .filter(Function.FunctionLanguage::isExternal).map(Function.FunctionLanguage::getName)
-                            .collect(Collectors.joining(" or "));
+                String lang = cfs.getLang();
+                if (lang == null) {
+                    throw new CompilationException(ErrorCode.COMPILATION_INCOMPATIBLE_FUNCTION_LANGUAGE, sourceLoc, "");
+                }
+                Function.FunctionLanguage functionLang;
+                try {
+                    functionLang = Function.FunctionLanguage.valueOf(lang.toUpperCase(Locale.ROOT));
+                } catch (IllegalArgumentException e) {
                     throw new CompilationException(ErrorCode.COMPILATION_INCOMPATIBLE_FUNCTION_LANGUAGE, sourceLoc,
-                            expectedExternalLanguages, cfs.getLang());
+                            lang);
+                }
+                if (functionLang.equals(getFunctionLanguage())) {
+                    throw new CompilationException(ErrorCode.COMPILATION_INCOMPATIBLE_FUNCTION_LANGUAGE, sourceLoc,
+                            lang);
                 }
                 Library libraryInMetadata = MetadataManager.INSTANCE.getLibrary(mdTxnCtx, dataverseName, libraryName);
                 if (libraryInMetadata == null) {
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.0.ddl.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.0.ddl.sqlpp
new file mode 100644
index 0000000..5a5bbec
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.0.ddl.sqlpp
@@ -0,0 +1,26 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+DROP DATAVERSE externallibtest if exists;
+CREATE DATAVERSE  externallibtest;
+USE externallibtest;
+
+create type CountryCapitalType if not exists as closed {
+country: string,
+capital: string
+};
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.1.lib.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.1.lib.sqlpp
new file mode 100644
index 0000000..d1e0e87
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.1.lib.sqlpp
@@ -0,0 +1,19 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+install externallibtest testlib target/data/externallib/asterix-external-data-testlib.zip
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.2.ddl.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.2.ddl.sqlpp
new file mode 100644
index 0000000..8439cfb
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.2.ddl.sqlpp
@@ -0,0 +1,29 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+use externallibtest;
+
+create function getCapital_default(a: string) returns CountryCapitalType
+language java as "testlib","org.apache.asterix.external.library.CapitalFinderFactory";
+
+create function getCapital_deterministic(a: string) returns CountryCapitalType
+language java deterministic as "testlib","org.apache.asterix.external.library.CapitalFinderFactory";
+
+create function getCapital_not_deterministic(a: string) returns CountryCapitalType
+language java not deterministic as "testlib","org.apache.asterix.external.library.CapitalFinderFactory";
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.3.query.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.3.query.sqlpp
new file mode 100644
index 0000000..4b2de03
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.3.query.sqlpp
@@ -0,0 +1,25 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+use externallibtest;
+
+{
+  "default": getCapital_default("United States").capital,
+  "deterministic": getCapital_deterministic("United States").capital,
+  "not_deterministic": getCapital_not_deterministic("United States").capital
+}
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.4.query.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.4.query.sqlpp
new file mode 100644
index 0000000..be157fd
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.4.query.sqlpp
@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/** Test that external functions declared as deterministic are constant folded,
+    while non-deterministic ones are not */
+
+use externallibtest;
+
+explain
+{
+  "default": getCapital_default("United States"),
+  "deterministic": getCapital_deterministic("United States"),
+  "not_deterministic": getCapital_not_deterministic("United States")
+}
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.5.lib.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.5.lib.sqlpp
new file mode 100644
index 0000000..86af80f
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.5.lib.sqlpp
@@ -0,0 +1,19 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+uninstall externallibtest testlib
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.6.ddl.sqlpp b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.6.ddl.sqlpp
new file mode 100644
index 0000000..cb57494
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/queries_sqlpp/external-library/deterministic/deterministic.6.ddl.sqlpp
@@ -0,0 +1,19 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+DROP DATAVERSE externallibtest;
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.3.adm b/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.3.adm
new file mode 100644
index 0000000..8eb8a8a
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.3.adm
@@ -0,0 +1 @@
+{ "default": "Washington D.C.", "deterministic": "Washington D.C.", "not_deterministic": "Washington D.C." }
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.4.adm b/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.4.adm
new file mode 100644
index 0000000..18dce53
--- /dev/null
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/results/external-library/deterministic/deterministic.4.adm
@@ -0,0 +1,8 @@
+distribute result [$$1]
+-- DISTRIBUTE_RESULT  |UNPARTITIONED|
+  exchange
+  -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+    assign [$$1] <- [{"default": getCapital_default("United States"), "deterministic": { country: "United States", capital: "Washington D.C." }, "not_deterministic": getCapital_not_deterministic("United States")}]
+    -- ASSIGN  |UNPARTITIONED|
+      empty-tuple-source
+      -- EMPTY_TUPLE_SOURCE  |UNPARTITIONED|
\ No newline at end of file
diff --git a/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_it_sqlpp.xml b/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_it_sqlpp.xml
index 532b8e8..31b1c13 100644
--- a/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_it_sqlpp.xml
+++ b/asterixdb/asterix-app/src/test/resources/runtimets/testsuite_it_sqlpp.xml
@@ -59,6 +59,11 @@
         <output-dir compare="Text">upperCase</output-dir>
       </compilation-unit>
     </test-case>
+    <test-case FilePath="external-library">
+      <compilation-unit name="deterministic">
+        <output-dir compare="Text">deterministic</output-dir>
+      </compilation-unit>
+    </test-case>
     <test-case FilePath="feeds">
       <compilation-unit name="feed-with-external-function">
         <output-dir compare="Text">feed-with-external-function</output-dir>
diff --git a/asterixdb/asterix-common/src/main/resources/asx_errormsg/en.properties b/asterixdb/asterix-common/src/main/resources/asx_errormsg/en.properties
index 257be8c..5dcc8ec 100644
--- a/asterixdb/asterix-common/src/main/resources/asx_errormsg/en.properties
+++ b/asterixdb/asterix-common/src/main/resources/asx_errormsg/en.properties
@@ -138,7 +138,7 @@
 1052 = Cannot create index with the same field \"%1$s\" specified more than once.
 1053 = Cannot create primary index on external dataset.
 1054 = Compilation failed due to some problem in the query plan.
-1055 = Incompatible function language. Expect %1$s, but %2$s found.
+1055 = Incompatible function language: %1$s.
 1056 = Too many options were specified for %1$s
 1057 = Expression of type %1$s is not supported in constant record
 1058 = Literal of type %1$s is not supported in constant record
diff --git a/asterixdb/asterix-lang-aql/src/main/java/org/apache/asterix/lang/aql/rewrites/AqlQueryRewriter.java b/asterixdb/asterix-lang-aql/src/main/java/org/apache/asterix/lang/aql/rewrites/AqlQueryRewriter.java
index a6fde0c..d94500d 100644
--- a/asterixdb/asterix-lang-aql/src/main/java/org/apache/asterix/lang/aql/rewrites/AqlQueryRewriter.java
+++ b/asterixdb/asterix-lang-aql/src/main/java/org/apache/asterix/lang/aql/rewrites/AqlQueryRewriter.java
@@ -129,7 +129,7 @@
         List<FunctionDecl> storedFunctionDecls = new ArrayList<>();
         for (Expression topLevelExpr : topStatement.getDirectlyEnclosedExpressions()) {
             storedFunctionDecls.addAll(FunctionUtil.retrieveUsedStoredFunctions(metadataProvider, topLevelExpr, funIds,
-                    null, expr -> getFunctionCalls(expr), func -> functionParser.getFunctionDecl(func),
+                    null, expr -> getFunctionCalls(expr), functionParser,
                     (signature, sourceLoc) -> CommonFunctionMapUtil.normalizeBuiltinFunctionSignature(signature)));
             declaredFunctions.addAll(storedFunctionDecls);
         }
diff --git a/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/parser/FunctionParser.java b/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/parser/FunctionParser.java
index 2a717a9..3a5d488 100644
--- a/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/parser/FunctionParser.java
+++ b/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/parser/FunctionParser.java
@@ -39,10 +39,14 @@
         this.parserFactory = parserFactory;
     }
 
+    public Function.FunctionLanguage getFunctionLanguage() {
+        return language;
+    }
+
     public FunctionDecl getFunctionDecl(Function function) throws CompilationException {
         if (!function.getLanguage().equals(language)) {
-            throw new CompilationException(ErrorCode.COMPILATION_INCOMPATIBLE_FUNCTION_LANGUAGE, language.getName(),
-                    function.getLanguage().getName());
+            throw new CompilationException(ErrorCode.COMPILATION_INCOMPATIBLE_FUNCTION_LANGUAGE, language,
+                    function.getLanguage());
         }
         IParser parser = parserFactory.createParser(new StringReader(function.getFunctionBody()));
         return parser.parseFunctionBody(function.getSignature(), function.getArgNames());
diff --git a/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/util/FunctionUtil.java b/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/util/FunctionUtil.java
index 5308b96..c9c8abc 100644
--- a/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/util/FunctionUtil.java
+++ b/asterixdb/asterix-lang-common/src/main/java/org/apache/asterix/lang/common/util/FunctionUtil.java
@@ -40,6 +40,7 @@
 import org.apache.asterix.lang.common.expression.TypeExpression;
 import org.apache.asterix.lang.common.expression.TypeReferenceExpression;
 import org.apache.asterix.lang.common.expression.UnorderedListTypeDefinition;
+import org.apache.asterix.lang.common.parser.FunctionParser;
 import org.apache.asterix.lang.common.statement.FunctionDecl;
 import org.apache.asterix.metadata.MetadataManager;
 import org.apache.asterix.metadata.MetadataTransactionContext;
@@ -118,11 +119,6 @@
     }
 
     @FunctionalInterface
-    public interface IFunctionParser {
-        FunctionDecl getFunctionDecl(Function function) throws CompilationException;
-    }
-
-    @FunctionalInterface
     public interface IFunctionNormalizer {
         FunctionSignature normalizeBuiltinFunctionSignature(FunctionSignature fs, SourceLocation sourceLoc)
                 throws CompilationException;
@@ -149,8 +145,8 @@
      */
     public static List<FunctionDecl> retrieveUsedStoredFunctions(MetadataProvider metadataProvider,
             Expression expression, List<FunctionSignature> declaredFunctions, List<FunctionDecl> inputFunctionDecls,
-            IFunctionCollector functionCollector, IFunctionParser functionParser,
-            IFunctionNormalizer functionNormalizer) throws CompilationException {
+            IFunctionCollector functionCollector, FunctionParser functionParser, IFunctionNormalizer functionNormalizer)
+            throws CompilationException {
         List<FunctionDecl> functionDecls =
                 inputFunctionDecls == null ? new ArrayList<>() : new ArrayList<>(inputFunctionDecls);
         if (expression == null) {
@@ -204,7 +200,7 @@
                         messageBuilder.toString());
             }
 
-            if (!function.getLanguage().isExternal()) {
+            if (functionParser.getFunctionLanguage().equals(function.getLanguage())) {
                 FunctionDecl functionDecl = functionParser.getFunctionDecl(function);
                 if (functionDecl != null) {
                     if (functionDecls.contains(functionDecl)) {
@@ -266,7 +262,7 @@
         return dependencies;
     }
 
-    private static Function lookupUserDefinedFunctionDecl(MetadataTransactionContext mdTxnCtx,
+    public static Function lookupUserDefinedFunctionDecl(MetadataTransactionContext mdTxnCtx,
             FunctionSignature signature) throws AlgebricksException {
         if (signature.getDataverseName() == null) {
             return null;
diff --git a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/SqlppQueryRewriter.java b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/SqlppQueryRewriter.java
index b439ba1..dc39425 100644
--- a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/SqlppQueryRewriter.java
+++ b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/SqlppQueryRewriter.java
@@ -91,7 +91,7 @@
     public static final String INLINE_WITH_OPTION = "inline_with";
     private static final boolean INLINE_WITH_OPTION_DEFAULT = true;
     private final IParserFactory parserFactory;
-    private final FunctionParser functionRepository;
+    private final FunctionParser functionParser;
     private IReturningStatement topExpr;
     private List<FunctionDecl> declaredFunctions;
     private LangRewritingContext context;
@@ -101,7 +101,7 @@
 
     public SqlppQueryRewriter(IParserFactory parserFactory) {
         this.parserFactory = parserFactory;
-        functionRepository = new FunctionParser(Function.FunctionLanguage.SQLPP, parserFactory);
+        functionParser = new FunctionParser(Function.FunctionLanguage.SQLPP, parserFactory);
     }
 
     protected void setup(List<FunctionDecl> declaredFunctions, IReturningStatement topExpr,
@@ -196,7 +196,8 @@
             return;
         }
         // Inlines with expressions.
-        InlineWithExpressionVisitor inlineWithExpressionVisitor = new InlineWithExpressionVisitor(context);
+        InlineWithExpressionVisitor inlineWithExpressionVisitor =
+                new InlineWithExpressionVisitor(context, metadataProvider);
         rewriteTopExpr(inlineWithExpressionVisitor, null);
     }
 
@@ -261,10 +262,10 @@
 
         List<FunctionDecl> usedStoredFunctionDecls = new ArrayList<>();
         for (Expression topLevelExpr : topExpr.getDirectlyEnclosedExpressions()) {
-            usedStoredFunctionDecls.addAll(FunctionUtil.retrieveUsedStoredFunctions(metadataProvider, topLevelExpr,
-                    funIds, null, expr -> getFunctionCalls(expr), func -> functionRepository.getFunctionDecl(func),
-                    (signature, sourceLoc) -> FunctionMapUtil.normalizeBuiltinFunctionSignature(signature, false,
-                            sourceLoc)));
+            usedStoredFunctionDecls
+                    .addAll(FunctionUtil.retrieveUsedStoredFunctions(metadataProvider, topLevelExpr, funIds, null,
+                            expr -> getFunctionCalls(expr), functionParser, (signature, sourceLoc) -> FunctionMapUtil
+                                    .normalizeBuiltinFunctionSignature(signature, false, sourceLoc)));
         }
         declaredFunctions.addAll(usedStoredFunctionDecls);
         if (inlineUdfs && !declaredFunctions.isEmpty()) {
diff --git a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/visitor/InlineWithExpressionVisitor.java b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/visitor/InlineWithExpressionVisitor.java
index 9865bdf..b46b9f9 100644
--- a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/visitor/InlineWithExpressionVisitor.java
+++ b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/rewrites/visitor/InlineWithExpressionVisitor.java
@@ -34,14 +34,15 @@
 import org.apache.asterix.lang.sqlpp.util.SqlppRewriteUtil;
 import org.apache.asterix.lang.sqlpp.visitor.CheckNonFunctionalExpressionVisitor;
 import org.apache.asterix.lang.sqlpp.visitor.base.AbstractSqlppExpressionScopingVisitor;
+import org.apache.asterix.metadata.declared.MetadataProvider;
 
 public class InlineWithExpressionVisitor extends AbstractSqlppExpressionScopingVisitor {
 
-    private final CheckNonFunctionalExpressionVisitor checkNonFunctionalExpressionVisitor =
-            new CheckNonFunctionalExpressionVisitor();
+    private final CheckNonFunctionalExpressionVisitor checkNonFunctionalExpressionVisitor;
 
-    public InlineWithExpressionVisitor(LangRewritingContext context) {
+    public InlineWithExpressionVisitor(LangRewritingContext context, MetadataProvider metadataProvider) {
         super(context);
+        checkNonFunctionalExpressionVisitor = new CheckNonFunctionalExpressionVisitor(metadataProvider);
     }
 
     @Override
diff --git a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/visitor/CheckNonFunctionalExpressionVisitor.java b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/visitor/CheckNonFunctionalExpressionVisitor.java
index 167660a..7a8f47f 100644
--- a/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/visitor/CheckNonFunctionalExpressionVisitor.java
+++ b/asterixdb/asterix-lang-sqlpp/src/main/java/org/apache/asterix/lang/sqlpp/visitor/CheckNonFunctionalExpressionVisitor.java
@@ -20,24 +20,45 @@
 package org.apache.asterix.lang.sqlpp.visitor;
 
 import org.apache.asterix.common.exceptions.CompilationException;
+import org.apache.asterix.common.exceptions.ErrorCode;
 import org.apache.asterix.common.functions.FunctionSignature;
 import org.apache.asterix.lang.common.expression.CallExpr;
 import org.apache.asterix.lang.common.util.FunctionUtil;
 import org.apache.asterix.lang.sqlpp.visitor.base.AbstractSqlppContainsExpressionVisitor;
+import org.apache.asterix.metadata.declared.MetadataProvider;
+import org.apache.asterix.metadata.entities.Function;
+import org.apache.hyracks.algebricks.common.exceptions.AlgebricksException;
 import org.apache.hyracks.algebricks.core.algebra.functions.IFunctionInfo;
 
 /**
  * Checks whether given expression is non-functional (i.e. whether it calls a non-functional function)
  */
 public final class CheckNonFunctionalExpressionVisitor extends AbstractSqlppContainsExpressionVisitor<Void> {
+
+    private final MetadataProvider metadataProvider;
+
+    public CheckNonFunctionalExpressionVisitor(MetadataProvider metadataProvider) {
+        this.metadataProvider = metadataProvider;
+    }
+
     @Override
     public Boolean visit(CallExpr callExpr, Void arg) throws CompilationException {
         FunctionSignature fs = callExpr.getFunctionSignature();
         IFunctionInfo fi = FunctionUtil.getBuiltinFunctionInfo(fs.getName(), fs.getArity());
-        // TODO: all external functions are considered functional for now.
-        // we'll need to revisit this code once we enable non-functional in ExternalFunctionInfo
-        if (fi != null && !fi.isFunctional()) {
-            return true;
+        if (fi != null) {
+            if (!fi.isFunctional()) {
+                return true;
+            }
+        } else {
+            try {
+                Function function =
+                        FunctionUtil.lookupUserDefinedFunctionDecl(metadataProvider.getMetadataTxnContext(), fs);
+                if (function != null && function.getDeterministic() != null && !function.getDeterministic()) {
+                    return true;
+                }
+            } catch (AlgebricksException e) {
+                throw new CompilationException(ErrorCode.METADATA_ERROR, e, callExpr.getSourceLocation());
+            }
         }
         return super.visit(callExpr, arg);
     }
diff --git a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entities/Function.java b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entities/Function.java
index 815cd22..f9328b8 100644
--- a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entities/Function.java
+++ b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entities/Function.java
@@ -22,7 +22,6 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
-import java.util.Locale;
 import java.util.Map;
 
 import org.apache.asterix.common.functions.FunctionSignature;
@@ -138,30 +137,11 @@
         return cache.dropFunction(this);
     }
 
+    // WARNING: These values are stored in function metadata. Do not rename.
     public enum FunctionLanguage {
-        // WARNING: do not change these language names because
-        // these values are stored in function metadata
-        AQL(false),
-        SQLPP(false),
-        JAVA(true),
-        PYTHON(true);
-
-        private final boolean isExternal;
-
-        FunctionLanguage(boolean isExternal) {
-            this.isExternal = isExternal;
-        }
-
-        public boolean isExternal() {
-            return isExternal;
-        }
-
-        public String getName() {
-            return name();
-        }
-
-        public static FunctionLanguage findByName(String name) {
-            return FunctionLanguage.valueOf(name.toUpperCase(Locale.ROOT));
-        }
+        AQL,
+        SQLPP,
+        JAVA,
+        PYTHON
     }
 }
diff --git a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entitytupletranslators/FunctionTupleTranslator.java b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entitytupletranslators/FunctionTupleTranslator.java
index 671fb46..65aa94c 100644
--- a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entitytupletranslators/FunctionTupleTranslator.java
+++ b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/entitytupletranslators/FunctionTupleTranslator.java
@@ -41,6 +41,8 @@
 import org.apache.asterix.builders.IARecordBuilder;
 import org.apache.asterix.builders.OrderedListBuilder;
 import org.apache.asterix.builders.RecordBuilder;
+import org.apache.asterix.common.exceptions.AsterixException;
+import org.apache.asterix.common.exceptions.ErrorCode;
 import org.apache.asterix.common.functions.FunctionSignature;
 import org.apache.asterix.common.metadata.DataverseName;
 import org.apache.asterix.common.transactions.TxnId;
@@ -126,9 +128,12 @@
                 .getValueByPos(MetadataRecordTypes.FUNCTION_ARECORD_FUNCTION_DEFINITION_FIELD_INDEX)).getStringValue();
         String languageValue = ((AString) functionRecord
                 .getValueByPos(MetadataRecordTypes.FUNCTION_ARECORD_FUNCTION_LANGUAGE_FIELD_INDEX)).getStringValue();
-        Function.FunctionLanguage language = Function.FunctionLanguage.findByName(languageValue);
-        if (language == null) {
-            throw new IllegalStateException(languageValue);
+
+        Function.FunctionLanguage language;
+        try {
+            language = Function.FunctionLanguage.valueOf(languageValue);
+        } catch (IllegalArgumentException e) {
+            throw new AsterixException(ErrorCode.METADATA_ERROR);
         }
         String functionKind =
                 ((AString) functionRecord.getValueByPos(MetadataRecordTypes.FUNCTION_ARECORD_FUNCTION_KIND_FIELD_INDEX))
@@ -311,7 +316,7 @@
 
         // write field 6
         fieldValue.reset();
-        aString.setValue(function.getLanguage().getName());
+        aString.setValue(function.getLanguage().name());
         stringSerde.serialize(aString, fieldValue.getDataOutput());
         recordBuilder.addField(MetadataRecordTypes.FUNCTION_ARECORD_FUNCTION_LANGUAGE_FIELD_INDEX, fieldValue);
 
diff --git a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalFunctionCompilerUtil.java b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalFunctionCompilerUtil.java
index 2469ff2..12c1c78 100644
--- a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalFunctionCompilerUtil.java
+++ b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalFunctionCompilerUtil.java
@@ -18,6 +18,8 @@
  */
 package org.apache.asterix.metadata.functions;
 
+import org.apache.asterix.common.exceptions.AsterixException;
+import org.apache.asterix.common.exceptions.ErrorCode;
 import org.apache.asterix.metadata.MetadataTransactionContext;
 import org.apache.asterix.metadata.entities.Function;
 import org.apache.asterix.om.typecomputer.base.IResultTypeComputer;
@@ -51,12 +53,16 @@
 
     private static IFunctionInfo getScalarFunctionInfo(MetadataTransactionContext txnCtx, Function function)
             throws AlgebricksException {
+        if (function.getDeterministic() == null) {
+            throw new AsterixException(ErrorCode.METADATA_ERROR);
+        }
+
         IAType returnType = function.getReturnType();
         IResultTypeComputer typeComputer = new ExternalTypeComputer(returnType, function.getArgTypes());
 
         return new ExternalScalarFunctionInfo(function.getSignature().createFunctionIdentifier(), returnType,
-                function.getFunctionBody(), function.getLanguage().getName(), function.getLibrary(),
-                function.getArgTypes(), function.getParams(), typeComputer);
+                function.getFunctionBody(), function.getLanguage().name(), function.getLibrary(),
+                function.getArgTypes(), function.getParams(), function.getDeterministic(), typeComputer);
     }
 
     private static IFunctionInfo getUnnestFunctionInfo(MetadataTransactionContext txnCtx, Function function) {
diff --git a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalScalarFunctionInfo.java b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalScalarFunctionInfo.java
index cdb9be5..a81bcf6 100644
--- a/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalScalarFunctionInfo.java
+++ b/asterixdb/asterix-metadata/src/main/java/org/apache/asterix/metadata/functions/ExternalScalarFunctionInfo.java
@@ -32,14 +32,15 @@
     private static final long serialVersionUID = 1L;
 
     public ExternalScalarFunctionInfo(String namespace, String library, String name, int arity, IAType returnType,
-            String body, String language, List<IAType> argumentTypes, Map<String, String> params,
+            String body, String language, List<IAType> argumentTypes, Map<String, String> params, boolean deterministic,
             IResultTypeComputer rtc) {
         super(namespace, name, arity, FunctionKind.SCALAR, argumentTypes, returnType, rtc, body, language, library,
-                params);
+                params, deterministic);
     }
 
     public ExternalScalarFunctionInfo(FunctionIdentifier fid, IAType returnType, String body, String language,
-            String library, List<IAType> argumentTypes, Map<String, String> params, IResultTypeComputer rtc) {
-        super(fid, FunctionKind.SCALAR, argumentTypes, returnType, rtc, body, language, library, params);
+            String library, List<IAType> argumentTypes, Map<String, String> params, boolean deterministic,
+            IResultTypeComputer rtc) {
+        super(fid, FunctionKind.SCALAR, argumentTypes, returnType, rtc, body, language, library, params, deterministic);
     }
 }
diff --git a/asterixdb/asterix-om/src/main/java/org/apache/asterix/om/functions/ExternalFunctionInfo.java b/asterixdb/asterix-om/src/main/java/org/apache/asterix/om/functions/ExternalFunctionInfo.java
index e76b07d..7220d05 100644
--- a/asterixdb/asterix-om/src/main/java/org/apache/asterix/om/functions/ExternalFunctionInfo.java
+++ b/asterixdb/asterix-om/src/main/java/org/apache/asterix/om/functions/ExternalFunctionInfo.java
@@ -41,16 +41,15 @@
 
     public ExternalFunctionInfo(String namespace, String name, int arity, FunctionKind kind, List<IAType> argumentTypes,
             IAType returnType, IResultTypeComputer rtc, String body, String language, String library,
-            Map<String, String> params) {
-        this(new FunctionIdentifier(namespace, name, arity), kind, argumentTypes, returnType, rtc, body, library,
-                language, params);
+            Map<String, String> params, boolean deterministic) {
+        this(new FunctionIdentifier(namespace, name, arity), kind, argumentTypes, returnType, rtc, body, language,
+                library, params, deterministic);
     }
 
     public ExternalFunctionInfo(FunctionIdentifier fid, FunctionKind kind, List<IAType> argumentTypes,
             IAType returnType, IResultTypeComputer rtc, String body, String language, String library,
-            Map<String, String> params) {
-        // TODO: fix CheckNonFunctionalExpressionVisitor once we have non-functional external functions
-        super(fid, true);
+            Map<String, String> params, boolean deterministic) {
+        super(fid, deterministic);
         this.rtc = rtc;
         this.argumentTypes = argumentTypes;
         this.body = body;