Issue #539: Allow override of builtin functions/aliases

h2/src/docsrc/help/help.csv

@@ -1165,6 +1165,19 @@ This setting is persistent.
 SET BINARY_COLLATION SIGNED
 "
 
+"Commands (Other)","SET BUILTIN_ALIAS_OVERRIDE","
+SET BUILTIN_ALIAS_OVERRIDE
+{ TRUE | FALSE } ] }
+","
+Allows the overriding of the builtin system date/time functions
+for unit testing purposes.
+
+Admin rights are required to execute this command.
+This command commits an open transaction in this connection.
+","
+SET BUILTIN_ALIAS_OVERRIDE TRUE
+"
+
 "Commands (Other)","SET COLLATION","
 SET [ DATABASE ] COLLATION
 { OFF | collationName [ STRENGTH { PRIMARY | SECONDARY | TERTIARY | IDENTICAL } ] }

h2/src/docsrc/html/changelog.html

@@ -21,6 +21,8 @@ Change Log
 
 <h2>Next Version (unreleased)</h2>
 <ul>
+<li>Issue #539: Allow override of builtin functions/aliases
+</li>
 <li>Issue #535: Allow explicit paths on Windows without drive letter
 </li>
 <li>Issue #549: Removed UNION ALL requirements for CTE

h2/src/main/org/h2/command/Parser.java

@@ -2707,10 +2707,17 @@ public class Parser {
         return function;
     }
 
-    private Function readFunctionWithoutParameters(String name) {
+    private Expression readFunctionWithoutParameters(String name) {
         if (readIf("(")) {
             read(")");
         }
+        if (database.isAllowBuiltinAliasOverride()) {
+            FunctionAlias functionAlias = database.getSchema(session.getCurrentSchemaName()).findFunction(name);
+            if (functionAlias != null) {
+                JavaFunction func = new JavaFunction(functionAlias, new Expression[0]);
+                return func;
+            }
+        }
         Function function = Function.getFunction(database, name);
         function.doneWithParameters();
         return function;
@@ -4850,8 +4857,18 @@ public class Parser {
 
     private CreateFunctionAlias parseCreateFunctionAlias(boolean force) {
         boolean ifNotExists = readIfNotExists();
-        String aliasName = readIdentifierWithSchema();
-        if (isKeyword(aliasName) ||
+        final boolean newAliasSameNameAsBuiltin = Function.getFunction(database, currentToken) != null;
+        String aliasName;
+        if (database.isAllowBuiltinAliasOverride() && newAliasSameNameAsBuiltin) {
+            aliasName = currentToken;
+            schemaName = session.getCurrentSchemaName();
+            read();
+        } else {
+            aliasName = readIdentifierWithSchema();
+        }
+        if (database.isAllowBuiltinAliasOverride() && newAliasSameNameAsBuiltin) {
+            // fine
+        } else if (isKeyword(aliasName) ||
                 Function.getFunction(database, aliasName) != null ||
                 getAggregateType(aliasName) >= 0) {
             throw DbException.get(ErrorCode.FUNCTION_ALIAS_ALREADY_EXISTS_1,

h2/src/main/org/h2/command/dml/Set.java

@@ -524,6 +524,16 @@ public class Set extends Prepared {
             session.setLazyQueryExecution(value == 1);
             break;
         }
+        case SetTypes.BUILTIN_ALIAS_OVERRIDE: {
+            session.getUser().checkAdmin();
+            int value = getIntValue();
+            if (value != 0 && value != 1) {
+                throw DbException.getInvalidValueException("BUILTIN_ALIAS_OVERRIDE",
+                        value);
+            }
+            database.setAllowBuiltinAliasOverride(value == 1);
+            break;
+        }
         default:
             DbException.throwInternalError("type="+type);
         }

h2/src/main/org/h2/command/dml/SetTypes.java

@@ -243,6 +243,11 @@ public class SetTypes {
      */
     public static final int LAZY_QUERY_EXECUTION = 46;
 
+    /**
+     * The type of SET BUILTIN_ALIAS_OVERRIDE statement.
+     */
+    public static final int BUILTIN_ALIAS_OVERRIDE = 47;
+
     private static final ArrayList<String> TYPES = New.arrayList();
 
     private SetTypes() {
@@ -298,6 +303,7 @@ public class SetTypes {
         list.add(BATCH_JOINS, "BATCH_JOINS");
         list.add(FORCE_JOIN_ORDER, "FORCE_JOIN_ORDER");
         list.add(LAZY_QUERY_EXECUTION, "LAZY_QUERY_EXECUTION");
+        list.add(BUILTIN_ALIAS_OVERRIDE, "BUILTIN_ALIAS_OVERRIDE");
     }
 
     /**

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

@@ -191,6 +191,7 @@ public class Database implements DataHandler {
     private int logMode;
     private MVTableEngine.Store mvStore;
     private int retentionTime;
+    private boolean allowBuiltinAliasOverride;
     private DbException backgroundException;
     private JavaObjectSerializer javaObjectSerializer;
     private String javaObjectSerializerName;
@@ -1983,7 +1984,15 @@ public class Database implements DataHandler {
             mvStore.getStore().setRetentionTime(value);
         }
     }
+    
+    public void setAllowBuiltinAliasOverride(boolean b) {
+        allowBuiltinAliasOverride = b;
+    }
 
+    public boolean isAllowBuiltinAliasOverride() {
+        return allowBuiltinAliasOverride;
+    }
+    
     /**
      * Check if flush-on-each-commit is enabled.
      *

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

@@ -76,6 +76,7 @@ public class TestFunctions extends TestBase implements AggregateFunction {
     @Override
     public void test() throws Exception {
         deleteDb("functions");
+        testOverrideAlias();
         testIfNull();
         testToDate();
         testToDateException();
@@ -1671,7 +1672,9 @@ public class TestFunctions extends TestBase implements AggregateFunction {
 
         conn.close();
     }
+
     private void testIfNull() throws SQLException {
+        deleteDb("functions");
         Connection conn = getConnection("functions");
         Statement stat = conn.createStatement(
                 ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
@@ -2113,6 +2116,26 @@ public class TestFunctions extends TestBase implements AggregateFunction {
         conn.close();
     }
 
+    private void testOverrideAlias()
+            throws SQLException, InterruptedException {
+        deleteDb("functions");
+        Connection conn = getConnection("functions");
+        conn.setAutoCommit(true);
+        Statement stat = conn.createStatement();
+
+        assertThrows(ErrorCode.FUNCTION_ALIAS_ALREADY_EXISTS_1, stat).execute("create alias CURRENT_TIMESTAMP for \"" +
+                getClass().getName() + ".currentTimestamp\"");
+
+        stat.execute("set BUILTIN_ALIAS_OVERRIDE true");
+        
+        stat.execute("create alias CURRENT_TIMESTAMP for \"" +
+                getClass().getName() + ".currentTimestampOverride\"");
+        
+        assertCallResult("3141", stat, "CURRENT_TIMESTAMP");
+        
+        conn.close();
+    }
+
     private void callCompiledFunction(String functionName) throws SQLException {
         deleteDb("functions");
         try (Connection conn = getConnection("functions")) {
@@ -2430,6 +2453,13 @@ public class TestFunctions extends TestBase implements AggregateFunction {
         }
         return new Object[] { buff.toString() };
     }
+    
+    /**
+     * This method is called via reflection from the database.
+     */
+    public static long currentTimestampOverride() {
+        return 3141;
+    }
 
     @Override
     public void add(Object value) {