planner: fold constant in projection elimination and TopN push down (#…

…14927)
pingcap · Feb 25, 2020 · 880a9b0 · 880a9b0
1 parent 88e0eaa
commit 880a9b0
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Showing 5 changed files with 20 additions and 14 deletions.
diff --git a/cmd/explaintest/r/explain_easy.result b/cmd/explaintest/r/explain_easy.result
@@ -506,17 +506,15 @@ EXPLAIN SELECT COUNT(1) FROM (SELECT COALESCE(b.region_name, '不详') region_na
 id	count	task	operator info
 StreamAgg_22	1.00	root	funcs:count(1)->Column#22
 └─HashAgg_25	1.00	root	group by:Column#32, Column#33, Column#34, funcs:firstrow(1)->Column#31
-  └─Projection_48	0.02	root	Column#14, Column#15, coalesce(test.test02.region_name, 不详)->Column#34
-    └─IndexMergeJoin_32	0.02	root	left outer join, inner:TableReader_30, outer key:Column#16, inner key:test.test02.id
-      ├─Union_37	0.02	root	
-      │ ├─Projection_38	0.01	root	test.test01.stat_date, test.test01.show_date, test.test01.region_id
-      │ │ └─TableReader_41	0.01	root	data:Selection_40
-      │ │   └─Selection_40	0.01	cop[tikv]	eq(test.test01.period, 1), ge(test.test01.stat_date, 20191202), ge(test.test01.stat_date, 20191202), gt(cast(0), 0), le(test.test01.stat_date, 20191202), le(test.test01.stat_date, 20191202)
-      │ │     └─TableFullScan_39	10000.00	cop[tikv]	table:test01, keep order:false, stats:pseudo
-      │ └─Projection_42	0.01	root	test.test01.stat_date, test.test01.show_date, test.test01.region_id
-      │   └─TableReader_45	0.01	root	data:Selection_44
-      │     └─Selection_44	0.01	cop[tikv]	eq(test.test01.period, 1), ge(test.test01.stat_date, 20191202), ge(test.test01.stat_date, 20191202), gt(cast(test.test01.registration_num), 0), le(test.test01.stat_date, 20191202), le(test.test01.stat_date, 20191202)
-      │       └─TableFullScan_43	10000.00	cop[tikv]	table:test01, keep order:false, stats:pseudo
+  └─Projection_46	0.01	root	Column#14, Column#15, coalesce(test.test02.region_name, 不详)->Column#34
+    └─IndexMergeJoin_32	0.01	root	left outer join, inner:TableReader_30, outer key:Column#16, inner key:test.test02.id
+      ├─Union_37	0.01	root	
+      │ ├─Projection_38	0.00	root	test.test01.stat_date, test.test01.show_date, test.test01.region_id
+      │ │ └─TableDual_39	0.00	root	rows:0
+      │ └─Projection_40	0.01	root	test.test01.stat_date, test.test01.show_date, test.test01.region_id
+      │   └─TableReader_43	0.01	root	data:Selection_42
+      │     └─Selection_42	0.01	cop[tikv]	eq(test.test01.period, 1), ge(test.test01.stat_date, 20191202), ge(test.test01.stat_date, 20191202), gt(cast(test.test01.registration_num), 0), le(test.test01.stat_date, 20191202), le(test.test01.stat_date, 20191202)
+      │       └─TableFullScan_41	10000.00	cop[tikv]	table:test01, keep order:false, stats:pseudo
       └─TableReader_30	1.00	root	data:TableRangeScan_29
         └─TableRangeScan_29	1.00	cop[tikv]	table:b, range: decided by [Column#16], keep order:true, stats:pseudo
 drop table if exists t;

diff --git a/cmd/explaintest/r/topn_pushdown.result b/cmd/explaintest/r/topn_pushdown.result
@@ -2,7 +2,7 @@ explain select * from ((select 4 as a) union all (select 33 as a)) tmp order by
 id	count	task	operator info
 TopN_17	1.00	root	Column#3:desc, offset:0, count:1
 └─Union_21	2.00	root	
-  ├─Projection_22	1.00	root	cast(4)->Column#3
+  ├─Projection_22	1.00	root	4->Column#3
   │ └─TableDual_23	1.00	root	rows:1
   └─Projection_24	1.00	root	33->Column#3
     └─TableDual_25	1.00	root	rows:1
diff --git a/planner/core/integration_test.go b/planner/core/integration_test.go
@@ -506,3 +506,11 @@ func (s *testIntegrationSuite) TestIndexMerge(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_eliminate_projection.go b/planner/core/rule_eliminate_projection.go
@@ -144,7 +144,7 @@ func (pe *projectionEliminator) eliminate(p LogicalPlan, replace map[string]*exp
 	if isProj {
 		if child, ok := p.Children()[0].(*LogicalProjection); ok && !ExprsHasSideEffects(child.Exprs) {
 			for i := range proj.Exprs {
-				proj.Exprs[i] = ReplaceColumnOfExpr(proj.Exprs[i], child, child.Schema())
+				proj.Exprs[i] = expression.FoldConstant(ReplaceColumnOfExpr(proj.Exprs[i], child, child.Schema()))
 			}
 			p.Children()[0] = child.Children()[0]
 		}

diff --git a/planner/core/rule_topn_push_down.go b/planner/core/rule_topn_push_down.go
@@ -114,7 +114,7 @@ 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.