Github user weijietong commented on a diff in the pull request:
https://github.com/apache/drill/pull/889#discussion_r140756869
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/planner/cost/DrillRelMdMaxRowCount.java
---
@@ -0,0 +1,42 @@
+/*
+ * 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
+ *
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+ */
+package org.apache.drill.exec.planner.cost;
+
+import org.apache.calcite.rel.core.TableScan;
+import org.apache.calcite.rel.metadata.ReflectiveRelMetadataProvider;
+import org.apache.calcite.rel.metadata.RelMdMaxRowCount;
+import org.apache.calcite.rel.metadata.RelMetadataProvider;
+import org.apache.calcite.rel.metadata.RelMetadataQuery;
+import org.apache.calcite.util.BuiltInMethod;
+import org.apache.drill.exec.planner.logical.DrillLimitRel;
+
+public class DrillRelMdMaxRowCount extends RelMdMaxRowCount {
+
+ private static final DrillRelMdMaxRowCount INSTANCE = new
DrillRelMdMaxRowCount();
+
+ public static final RelMetadataProvider SOURCE =
ReflectiveRelMetadataProvider.reflectiveSource(BuiltInMethod.MAX_ROW_COUNT.method,
INSTANCE);
+
+ public Double getMaxRowCount(DrillLimitRel rel, RelMetadataQuery mq) {
+ return rel.getRows();
+ }
+
+ @Override
+ public Double getMaxRowCount(TableScan rel, RelMetadataQuery mq) {
+ return rel.getRows();
--- End diff --
@amansinha100 Your assumption is right. My initial motivation is to
materialize the count distinct value of one table column without the group by
clause into a single-row table i.e. a cache. To accelerate users' query time,
this materialization seems reasonable.
Since getMaxRowCount() only invoked by the `JoinUtils.isScalarSubquery`
,there's no potential issues.
---
