planner: fold constant in TopN push down (#14944)

pingcap · Feb 26, 2020 · 094d6b5 · 094d6b5
1 parent 037b369
commit 094d6b5
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Showing 2 changed files with 9 additions and 1 deletion.
diff --git a/planner/core/integration_test.go b/planner/core/integration_test.go
@@ -286,3 +286,11 @@ func (s *testIntegrationSuite) TestMaxMinEliminate(c *C) {
 		tk.MustQuery(tt).Check(testkit.Rows(output[i].Plan...))
 	}
 }
+
+func (s *testIntegrationSuite) TestTopNByConstFunc(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustQuery("select max(t.col) from (select 'a' as col union all select '' as col) as t").Check(testkit.Rows(
+		"a",
+	))
+}
diff --git a/planner/core/rule_topn_push_down.go b/planner/core/rule_topn_push_down.go
@@ -109,7 +109,7 @@ func (p *LogicalUnionAll) pushDownTopN(topN *LogicalTopN) LogicalPlan {
 func (p *LogicalProjection) pushDownTopN(topN *LogicalTopN) LogicalPlan {
 	if topN != nil {
 		for _, by := range topN.ByItems {
-			by.Expr = expression.ColumnSubstitute(by.Expr, p.schema, p.Exprs)
+			by.Expr = expression.FoldConstant(expression.ColumnSubstitute(by.Expr, p.schema, p.Exprs))
 		}
 
 		// remove meaningless constant sort items.