planner: skip pure-constant generated column substitution (#67473)

close #66859

Author: hawkingrei

pkg/planner/core/casetest/join/join_test.go

@@ -320,5 +320,14 @@ JOIN
 			`      └─Selection cop[tikv]  not(isnull(test.t3_issue60076.b))`,
 			`        └─TableFullScan cop[tikv] table:t3_issue60076 keep order:false, stats:pseudo`))
 		tk.MustQuery(`show warnings`).Check(testkit.Rows())
+
+		tk.MustExec(`drop table if exists issue66859_t0, issue66859_t1, issue66859_t2`)
+		tk.MustExec(`create table issue66859_t0(c0 int)`)
+		tk.MustExec(`create table issue66859_t1 like issue66859_t0`)
+		tk.MustExec(`create index i0 on issue66859_t1((5))`)
+		tk.MustExec(`insert into issue66859_t0(c0) values(-1)`)
+		tk.MustQuery(`select /* issue:66859 */ issue66859_t0.c0 as ref0, issue66859_t1.c0 as ref2
+			from issue66859_t0 left join issue66859_t1 on issue66859_t0.c0 = issue66859_t1.c0
+			where 5 >= issue66859_t0.c0`).Check(testkit.Rows("-1 <nil>"))
 	})
 }

pkg/planner/core/rule_generate_column_substitute.go

@@ -81,6 +81,12 @@ func collectGenerateColumn(lp base.LogicalPlan, exprToColumn ExprColumnMap) {
 				s := ds.Schema().Columns
 				col := expression.ColInfo2Col(s, colInfo)
 				if col != nil && col.GetType(ectx).PartialEqual(col.VirtualExpr.GetType(ectx), lp.SCtx().GetSessionVars().EnableUnsafeSubstitute) {
+					// Replacing a literal with a generated column that is itself a pure constant is not
+					// semantically neutral across outer joins, because null-augmentation can turn the
+					// generated column into NULL while the original literal stays constant.
+					if len(expression.ExtractColumns(col.VirtualExpr)) == 0 {
+						continue
+					}
 					exprToColumn[col.VirtualExpr] = col
 				}
 			}