Github user amansinha100 commented on a diff in the pull request:
https://github.com/apache/drill/pull/889#discussion_r140160556
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/planner/cost/DrillRelMdMaxRowCount.java
---
@@ -0,0 +1,42 @@
+/*
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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 --
Sorry, I had been meaning to reply sooner. I think overloading the
getMaxRowCount() to return rel.getRows() can create potential issues...because
getMaxRowCount() should always return whatever is the *maximum* row count
possible for that RelNode. Here, if you return TableScan.getRows(), the value
is an *estimate*, which means in reality it could be higher. The caller might
make incorrect decision based on this value.
I am thinking about your original motivation for the changes. Are you
materializing the results into a single-row table ? It sounds like you want a
special table scan whose max row count is 1. Is materializing the only
option ? (the reason I am curious is it is odd to materialize very small data
sets such as 1 row).
---
