planner: not use the latest schema when plan-cache is off

go.sum

@@ -524,6 +524,7 @@ github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQD
 github.com/sasha-s/go-deadlock v0.2.0/go.mod h1:StQn567HiB1fF2yJ44N9au7wOhrPS3iZqiDbRupzT10=
 github.com/satori/go.uuid v1.2.0/go.mod h1:dA0hQrYB0VpLJoorglMZABFdXlWrHn1NEOzdhQKdks0=
 github.com/sergi/go-diff v1.0.1-0.20180205163309-da645544ed44/go.mod h1:0CfEIISq7TuYL3j771MWULgwwjU+GofnZX9QAmXWZgo=
+github.com/sergi/go-diff v1.1.0 h1:we8PVUC3FE2uYfodKH/nBHMSetSfHDR6scGdBi+erh0=
 github.com/sergi/go-diff v1.1.0/go.mod h1:STckp+ISIX8hZLjrqAeVduY0gWCT9IjLuqbuNXdaHfM=
 github.com/shirou/gopsutil v2.19.10+incompatible/go.mod h1:5b4v6he4MtMOwMlS0TUMTu2PcXUg8+E1lC7eC3UO/RA=
 github.com/shirou/gopsutil v3.20.12+incompatible h1:6VEGkOXP/eP4o2Ilk8cSsX0PhOEfX6leqAnD+urrp9M=

planner/optimize.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/pingcap/errors"
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/parser"
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/tidb/bindinfo"
@@ -238,12 +239,16 @@ func optimize(ctx context.Context, sctx sessionctx.Context, node ast.Node, is in
 	sctx.GetSessionVars().RewritePhaseInfo.DurationRewrite = time.Since(beginRewrite)
 
 	if execPlan, ok := p.(*plannercore.Execute); ok {
+		failpoint.Inject("optimizeExecuteStmt", nil)
 		execID := execPlan.ExecID
 		if execPlan.Name != "" {
 			execID = sctx.GetSessionVars().PreparedStmtNameToID[execPlan.Name]
 		}
 		if preparedPointer, ok := sctx.GetSessionVars().PreparedStmts[execID]; ok {
-			if preparedObj, ok := preparedPointer.(*core.CachedPrepareStmt); ok && preparedObj.ForUpdateRead {
+			// When plan cache is enabled, and it's a for-update read, use the latest schema to detect plan expired.
+			// Otherwise, do not update schema to avoid plan and execution use different schema versions.
+			if preparedObj, ok := preparedPointer.(*core.CachedPrepareStmt); ok && preparedObj.ForUpdateRead &&
+				preparedObj.PreparedAst.UseCache {
 				is = domain.GetDomain(sctx).InfoSchema()
 			}
 		}

session/pessimistic_test.go

@@ -2623,3 +2623,31 @@ func (s *testPessimisticSuite) TestChangeLockToPut(c *C) {
 
 	tk.MustExec("admin check table t1")
 }
+
+func (s *testPessimisticSuite) TestIssue30940(c *C) {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	tk2 := testkit.NewTestKitWithInit(c, s.store)
+
+	tk.MustExec("use test")
+	tk2.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(id int primary key, v int)")
+	tk.MustExec("insert into t values(1, 1)")
+
+	stmts := []string{
+		"update t set v = v + 1 where id = 1",
+		"delete from t where id = 1",
+	}
+	errCh := make(chan error, 1)
+	for _, stmt := range stmts {
+		tk.MustExec(fmt.Sprintf("prepare t from '%s'", stmt))
+		tk2.MustExec("alter table t add column a int")
+		c.Assert(failpoint.Enable("github.com/pingcap/tidb/planner/optimizeExecuteStmt", "pause"), IsNil)
+		go func() {
+			errCh <- tk.ExecToErr("execute t")
+		}()
+		tk2.MustExec("alter table t drop column a")
+		c.Assert(failpoint.Disable("github.com/pingcap/tidb/planner/optimizeExecuteStmt"), IsNil)
+		c.Assert(<-errCh, IsNil)
+	}
+}