*: replace compareDatum by compare and ignore warnings is collate is empty

expression/constant.go

@@ -25,6 +25,7 @@ import (
 	"github.com/pingcap/tidb/types/json"
 	"github.com/pingcap/tidb/util/chunk"
 	"github.com/pingcap/tidb/util/codec"
+	"github.com/pingcap/tidb/util/collate"
 )
 
 // NewOne stands for a number 1.
@@ -345,7 +346,7 @@ func (c *Constant) Equal(ctx sessionctx.Context, b Expression) bool {
 	if err1 != nil || err2 != nil {
 		return false
 	}
-	con, err := c.Value.CompareDatum(ctx.GetSessionVars().StmtCtx, &y.Value)
+	con, err := c.Value.Compare(ctx.GetSessionVars().StmtCtx, &y.Value, collate.GetBinaryCollator())
 	if err != nil || con != 0 {
 		return false
 	}

expression/constant_propagation.go

@@ -21,6 +21,7 @@ import (
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
+	"github.com/pingcap/tidb/util/collate"
 	"github.com/pingcap/tidb/util/disjointset"
 	"github.com/pingcap/tidb/util/logutil"
 	"go.uber.org/zap"
@@ -59,7 +60,8 @@ func (s *basePropConstSolver) tryToUpdateEQList(col *Column, con *Constant) (boo
 	id := s.getColID(col)
 	oldCon := s.eqList[id]
 	if oldCon != nil {
-		return false, !oldCon.Equal(s.ctx, con)
+		res, err := oldCon.Value.Compare(s.ctx.GetSessionVars().StmtCtx, &con.Value, collate.GetCollator(col.GetType().Collate))
+		return false, res != 0 || err != nil
 	}
 	s.eqList[id] = con
 	return true, false

expression/integration_test.go

@@ -6832,6 +6832,7 @@ func (s *testIntegrationSerialSuite) TestCollateConstantPropagation(c *C) {
 	tk.MustExec("insert into t2 values ('A');")
 	tk.MustExec("set names utf8 collate utf8_general_ci;")
 	tk.MustQuery("select * from t1, t2 where t1.a=t2.a and t1.a= 'a';").Check(testkit.Rows("a A"))
+	tk.MustQuery("select * from t1 where a='a' and a = 'A'").Check(testkit.Rows("a A"))
 }
 
 func (s *testIntegrationSuite2) TestIssue17791(c *C) {

expression/partition_pruner.go

@@ -19,6 +19,7 @@ import (
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
+	"github.com/pingcap/tidb/util/collate"
 	"github.com/pingcap/tidb/util/disjointset"
 )
 
@@ -60,8 +61,8 @@ func (p *hashPartitionPruner) reduceColumnEQ() bool {
 		father := p.unionSet.FindRoot(i)
 		if p.constantMap[i] != nil {
 			if p.constantMap[father] != nil {
-				// May has conflict here.
-				if !p.constantMap[father].Equal(p.ctx, p.constantMap[i]) {
+				// May has conflict here. We can choose collation from lhs or rhs, they should be equal. Exception is that `NULL` values.
+				if eq, err := p.constantMap[father].Value.Compare(p.ctx.GetSessionVars().StmtCtx, &p.constantMap[i].Value, collate.GetCollator(p.constantMap[i].GetType().Collate)); eq != 0 || err != nil {
 					return true
 				}
 			} else {
@@ -95,7 +96,8 @@ func (p *hashPartitionPruner) reduceConstantEQ() bool {
 		if col != nil {
 			id := p.getColID(col)
 			if p.constantMap[id] != nil {
-				if p.constantMap[id].Equal(p.ctx, cond) {
+				// We can choose collation from lhs or rhs, they should be equal. Exception is that `NULL` values.
+				if eq, err := p.constantMap[id].Value.Compare(p.ctx.GetSessionVars().StmtCtx, &cond.Value, collate.GetCollator(cond.GetType().Collate)); eq == 0 && err == nil {
 					continue
 				}
 				return true

util/collate/collate.go

@@ -143,11 +143,13 @@ func GetCollator(collate string) Collator {
 	if atomic.LoadInt32(&newCollationEnabled) == 1 {
 		ctor, ok := newCollatorMap[collate]
 		if !ok {
-			logutil.BgLogger().Warn(
-				"Unable to get collator by name, use binCollator instead.",
-				zap.String("name", collate),
-				zap.Stack("stack"))
-			return newCollatorMap["utf8mb4_bin"]
+			if collate != "" {
+				logutil.BgLogger().Warn(
+					"Unable to get collator by name, use binCollator instead.",
+					zap.String("name", collate),
+					zap.Stack("stack"))
+			}
+			return newCollatorMap[charset.CollationUTF8MB4]
 		}
 		return ctor
 	}