server: use stmt should apply to subsequent stmts in multi-stmt mode (#26905)

server/conn.go

@@ -1622,6 +1622,13 @@ func (cc *clientConn) prefetchPointPlanKeys(ctx context.Context, stmts []ast.Stm
 	is := domain.GetDomain(cc.ctx).InfoSchema()
 	sc := vars.StmtCtx
 	for i, stmt := range stmts {
+		switch stmt.(type) {
+		case *ast.UseStmt:
+			// If there is a "use db" statement, we shouldn't cache even if it's possible.
+			// Consider the scenario where there are statements that could execute on multiple
+			// schemas, but the schema is actually different.
+			return nil, nil
+		}
 		// TODO: the preprocess is run twice, we should find some way to avoid do it again.
 		if err = plannercore.Preprocess(cc.ctx, stmt, is); err != nil {
 			return nil, err

server/server_test.go

@@ -1728,6 +1728,25 @@ func (cli *testServerClient) runTestMultiStatements(c *C) {
 		} else {
 			dbt.Error("no data")
 		}
+
+		// Test issue #26688
+		// First we "reset" the CurrentDB by using a database and then dropping it.
+		dbt.mustExec("CREATE DATABASE dropme")
+		dbt.mustExec("USE dropme")
+		dbt.mustExec("DROP DATABASE dropme")
+		var usedb string
+		rows = dbt.mustQuery("SELECT IFNULL(DATABASE(),'success')")
+		if rows.Next() {
+			err = rows.Scan(&usedb)
+			c.Assert(err, IsNil)
+			c.Assert(usedb, Equals, "success")
+		} else {
+			dbt.Error("no database() result")
+		}
+		// Because no DB is selected, if the use multistmtuse is not successful, then
+		// the create table + drop table statements will return errors.
+		dbt.mustExec("CREATE DATABASE multistmtuse")
+		dbt.mustExec("use multistmtuse; create table if not exists t1 (id int); drop table t1;")
 	})
 }