Add a single-value cache for DETERMINISTIC ALIAS method calls, reduces unnecessary calls

h2/src/main/org/h2/engine/FunctionAlias.java

@@ -279,7 +279,7 @@ public class FunctionAlias extends SchemaObjectBase {
      * Each method must have a different number of parameters however.
      * This helper class represents one such method.
      */
-    public static class JavaMethod implements Comparable<JavaMethod> {
+    public class JavaMethod implements Comparable<JavaMethod> {
         private final int id;
         private final Method method;
         private final int dataType;
@@ -287,6 +287,11 @@ public class FunctionAlias extends SchemaObjectBase {
         private boolean varArgs;
         private Class<?> varArgClass;
         private int paramCount;
+        /**
+         * Cache the value of the last call if the function is deterministic.
+         */
+        private Object[] previousParams;
+        private Value previousReturnValue;
 
         JavaMethod(Method method, int id) {
             this.method = method;
@@ -398,6 +403,11 @@ public class FunctionAlias extends SchemaObjectBase {
                     params[p] = o;
                 }
             }
+            if (deterministic) {
+                if (previousParams != null && Arrays.deepEquals(previousParams, params)) {
+                    return previousReturnValue;
+                }
+            }
             boolean old = session.getAutoCommit();
             Value identity = session.getLastScopeIdentity();
             boolean defaultConnection = session.getDatabase().getSettings().defaultConnection;
@@ -409,9 +419,6 @@ public class FunctionAlias extends SchemaObjectBase {
                         Driver.setDefaultConnection(session.createConnection(columnList));
                     }
                     returnValue = method.invoke(null, params);
-                    if (returnValue == null) {
-                        return ValueNull.INSTANCE;
-                    }
                 } catch (InvocationTargetException e) {
                     StatementBuilder buff = new StatementBuilder(method.getName());
                     buff.append('(');
@@ -424,11 +431,21 @@ public class FunctionAlias extends SchemaObjectBase {
                 } catch (Exception e) {
                     throw DbException.convert(e);
                 }
+                Value ret;
+                if (returnValue == null) {
+                    ret = ValueNull.INSTANCE;
+                } else {
                     if (Value.class.isAssignableFrom(method.getReturnType())) {
-                    return (Value) returnValue;
+                        ret = (Value) returnValue;
+                    } else {
+                        ret = DataType.convertToValue(session, returnValue, dataType);
+                    }
+                }
+                if (deterministic) {
+                    previousParams = params;
+                    previousReturnValue = ret;
                 }
-                Value ret = DataType.convertToValue(session, returnValue, dataType);
-                return ret.convertTo(dataType);
+                return ret;
             } finally {
                 session.setLastScopeIdentity(identity);
                 session.setAutoCommit(old);

h2/src/test/org/h2/test/db/TestFunctions.java

@@ -80,6 +80,7 @@ public class TestFunctions extends TestBase implements AggregateFunction {
         testNvl2();
         testConcatWs();
         testTruncate();
+        testCachingOfDeterministicFunctionAlias();
         deleteDb("functions");
         FileUtils.deleteRecursive(TEMP_DIR, true);
     }
@@ -929,6 +930,31 @@ public class TestFunctions extends TestBase implements AggregateFunction {
         conn.close();
     }
 
+    private static int countOfCallsToTestCache;
+    
+    private void testCachingOfDeterministicFunctionAlias() throws SQLException {
+        deleteDb("functions");
+        Connection conn = getConnection("functions");
+        Statement stat = conn.createStatement();
+        stat.execute("create alias MYTF2 deterministic for \"" + TestFunctions.class.getName() + ".testCache\"");
+        stat.execute("create view MVIEW2 as select * from MYTF2()");
+        countOfCallsToTestCache = 0;
+        stat.executeQuery("select * from MVIEW2");
+        assertEquals(0, countOfCallsToTestCache);
+        conn.close();
+    }
+
+    /**
+     * This method is called via reflection from the database.
+     */
+    public static synchronized ResultSet testCache() {
+        countOfCallsToTestCache++;
+        SimpleResultSet rs = new SimpleResultSet();
+        rs.addColumn("ID", Types.INTEGER, 10, 0);
+        rs.addRow(0);
+        return rs;
+    }
+    
     private void assertCallResult(String expected, Statement stat, String sql) throws SQLException {
         ResultSet rs = stat.executeQuery("CALL " + sql);
         rs.next();