planner: fix the coercibility of the cast function (#21705)

expression/collation.go

@@ -91,7 +91,7 @@ const (
 	CoercibilityExplicit Coercibility = 0
 	// CoercibilityNone is derived from the concatenation of two strings with different collations.
 	CoercibilityNone Coercibility = 1
-	// CoercibilityImplicit is derived from a column or a stored routine parameter or local variable.
+	// CoercibilityImplicit is derived from a column or a stored routine parameter or local variable or cast() function.
 	CoercibilityImplicit Coercibility = 2
 	// CoercibilitySysconst is derived from a “system constant” (the string returned by functions such as USER() or VERSION()).
 	CoercibilitySysconst Coercibility = 3

expression/integration_test.go

@@ -7458,6 +7458,14 @@ func (s *testIntegrationSerialSuite) TestCollationIndexJoin(c *C) {
 	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1815 Optimizer Hint /*+ INL_MERGE_JOIN(t2) */ is inapplicable"))
 }
 
+func (s *testIntegrationSuite2) TestCastCoer(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	tk.MustQuery("select coercibility(binary('a'))").Check(testkit.Rows("2"))
+	tk.MustQuery("select coercibility(cast('a' as char(10)))").Check(testkit.Rows("2"))
+	tk.MustQuery("select coercibility(convert('abc', char(10)));").Check(testkit.Rows("2"))
+}
+
 func (s *testIntegrationSuite) TestDatetimeUserVariable(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("set @p = now()")

planner/core/expression_rewriter.go

@@ -1040,6 +1040,10 @@ func (er *expressionRewriter) Leave(originInNode ast.Node) (retNode ast.Node, ok
 			return retNode, false
 		}
 
+		if v.Tp.EvalType() == types.ETString {
+			arg.SetCoercibility(expression.CoercibilityImplicit)
+		}
+
 		er.ctxStack[len(er.ctxStack)-1] = expression.BuildCastFunction(er.sctx, arg, v.Tp)
 		er.ctxNameStk[len(er.ctxNameStk)-1] = types.EmptyName
 	case *ast.PatternLikeExpr: