Expressions: Optimization for string expressions on the __time column.

benchmarks/src/main/java/io/druid/benchmark/ExpressionSelectorBenchmark.java

@@ -124,6 +124,80 @@ public void tearDown() throws Exception
     }
   }
 
+  @Benchmark
+  public void timeFormatUsingExpression(Blackhole blackhole) throws Exception
+  {
+    final Sequence<Cursor> cursors = new QueryableIndexStorageAdapter(index).makeCursors(
+        null,
+        index.getDataInterval(),
+        VirtualColumns.create(
+            ImmutableList.of(
+                new ExpressionVirtualColumn(
+                    "v",
+                    "timestamp_format(__time, 'yyyy-MM-dd')",
+                    ValueType.LONG,
+                    TestExprMacroTable.INSTANCE
+                )
+            )
+        ),
+        Granularities.ALL,
+        false,
+        null
+    );
+
+    final List<?> results = Sequences.toList(
+        Sequences.map(
+            cursors,
+            cursor -> {
+              final DimensionSelector selector = cursor.getColumnSelectorFactory().makeDimensionSelector(
+                  new DefaultDimensionSpec("v", "v", ValueType.STRING)
+              );
+              consumeDimension(cursor, selector, blackhole);
+              return null;
+            }
+        ),
+        new ArrayList<>()
+    );
+
+    blackhole.consume(results);
+  }
+
+  @Benchmark
+  public void timeFormatUsingExtractionFn(Blackhole blackhole) throws Exception
+  {
+    final Sequence<Cursor> cursors = new QueryableIndexStorageAdapter(index).makeCursors(
+        null,
+        index.getDataInterval(),
+        VirtualColumns.EMPTY,
+        Granularities.ALL,
+        false,
+        null
+    );
+
+    final List<?> results = Sequences.toList(
+        Sequences.map(
+            cursors,
+            cursor -> {
+              final DimensionSelector selector = cursor
+                  .getColumnSelectorFactory()
+                  .makeDimensionSelector(
+                      new ExtractionDimensionSpec(
+                          Column.TIME_COLUMN_NAME,
+                          "v",
+                          new TimeFormatExtractionFn("yyyy-MM-dd", null, null, null, false)
+                      )
+                  );
+
+              consumeDimension(cursor, selector, blackhole);
+              return null;
+            }
+        ),
+        new ArrayList<>()
+    );
+
+    blackhole.consume(results);
+  }
+
   @Benchmark
   public void timeFloorUsingExpression(Blackhole blackhole) throws Exception
   {

processing/src/main/java/io/druid/segment/virtual/ExpressionSelectors.java

@@ -176,9 +176,16 @@ public static DimensionSelector makeDimensionSelector(
       final String column = Iterables.getOnlyElement(columns);
       final ColumnCapabilities capabilities = columnSelectorFactory.getColumnCapabilities(column);
 
-      if (capabilities != null
-          && capabilities.getType() == ValueType.STRING
-          && capabilities.isDictionaryEncoded()) {
+      if (column.equals(Column.TIME_COLUMN_NAME)) {
+        // Optimization for expressions that hit the __time column and nothing else.
+        // May be worth applying this optimization to all long columns?
+        return new SingleLongInputCachingExpressionDimensionSelector(
+            columnSelectorFactory.makeColumnValueSelector(Column.TIME_COLUMN_NAME),
+            expression
+        );
+      } else if (capabilities != null
+                 && capabilities.getType() == ValueType.STRING
+                 && capabilities.isDictionaryEncoded()) {
         // Optimization for dimension selectors that wrap a single underlying string column.
         return new SingleStringInputDimensionSelector(
             columnSelectorFactory.makeDimensionSelector(new DefaultDimensionSpec(column, column, ValueType.STRING)),

processing/src/main/java/io/druid/segment/virtual/SingleLongInputCachingExpressionDimensionSelector.java

@@ -0,0 +1,85 @@
+/*
+ * Licensed to Metamarkets Group Inc. (Metamarkets) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. Metamarkets 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.
+ */
+
+package io.druid.segment.virtual;
+
+import com.google.common.base.Preconditions;
+import io.druid.java.util.common.ISE;
+import io.druid.math.expr.Expr;
+import io.druid.math.expr.Parser;
+import io.druid.query.monomorphicprocessing.RuntimeShapeInspector;
+import io.druid.segment.BaseLongColumnValueSelector;
+
+public class SingleLongInputCachingExpressionDimensionSelector extends BaseSingleValueDimensionSelector
+{
+  private final BaseLongColumnValueSelector selector;
+  private final Expr expression;
+  private final SingleInputBindings bindings = new SingleInputBindings();
+
+  // Is this the first row?
+  private boolean first = true;
+
+  // Last read input value
+  private long lastInput;
+
+  // Last computed output value
+  private String lastOutput;
+
+  public SingleLongInputCachingExpressionDimensionSelector(
+      final BaseLongColumnValueSelector selector,
+      final Expr expression
+  )
+  {
+    // Verify expression has just one binding.
+    if (Parser.findRequiredBindings(expression).size() != 1) {
+      throw new ISE("WTF?! Expected expression with just one binding");
+    }
+
+    this.selector = Preconditions.checkNotNull(selector, "selector");
+    this.expression = Preconditions.checkNotNull(expression, "expression");
+  }
+
+  @Override
+  public void inspectRuntimeShape(final RuntimeShapeInspector inspector)
+  {
+    inspector.visit("selector", selector);
+    inspector.visit("expression", expression);
+  }
+
+  @Override
+  public String getValue()
+  {
+    final long currentInput = selector.getLong();
+
+    if (first || lastInput != currentInput) {
+      final String output = eval(currentInput);
+      lastInput = currentInput;
+      lastOutput = output;
+      first = false;
+    }
+
+    return lastOutput;
+  }
+
+  private String eval(final long value)
+  {
+    bindings.set(value);
+    return expression.eval(bindings).asString();
+  }
+}