Github user amansinha100 commented on a diff in the pull request:
https://github.com/apache/drill/pull/905#discussion_r140417433
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/planner/cost/DrillRelMdMaxRowCount.java
---
@@ -0,0 +1,71 @@
+/*
+* 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.
+*/
+package org.apache.drill.exec.planner.cost;
+
+import org.apache.calcite.plan.volcano.RelSubset;
+import org.apache.calcite.rel.SingleRel;
+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.physical.AbstractPrel;
+import org.apache.drill.exec.planner.physical.ScanPrel;
+
+/**
+ * DrillRelMdMaxRowCount supplies a specific implementation of
+ * {@link RelMetadataQuery#getMaxRowCount} for Drill.
+ */
+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(ScanPrel rel, RelMetadataQuery mq) {
+ // the actual row count is known so returns its value
+ return rel.estimateRowCount(mq);
--- End diff --
Returning 'estimated' row count means that this is just an estimate, not
the actual value which could be higher. Looking at the implementation of
estimatedRowCount() for several of the storage/format plugins, there are
several that use NO_EXACT_ROW_COUNT. for instance see [1] for the text format
plugin. So, I feel overloading getMaxRowCount() to return an estimate may
cause problems. If you look at the semantics of getMaxRowCount in Calcite's
RelMdMaxRowCount, it is only intended for cases where **_during planning
time_** we can guarantee that the max row count will never exceed that value.
For example, an Aggregate with no group-by clause or a LIMIT etc.
[1]
https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/store/dfs/easy/EasyFormatPlugin.java#L186
---
