different behavior between mysql and tidb about use clause in transaction

[darren]

Bug Report

1. Minimal reproduce step (Required)

The actual problem comes from the syncer tool which is deprecated now, we have not switched to DM yet.

package main

import (
        "database/sql"
        "log"

        _ "github.com/go-sql-driver/mysql"
)

func main() {
        db, err := sql.Open("mysql", "root@tcp(127.0.0.1:4000)/?multiStatements=true")
        if err != nil {
                log.Fatalf("open mysql err: %s", err)
        }

        txn, err := db.Begin()
        if err != nil {
                log.Fatal(err)
        }

        _, err = txn.Exec("use test; create table if not exists t1 (id int); drop table t1;")
        if err != nil {
                log.Fatal(err)
        }

        err = txn.Commit()
        if err != nil {
                log.Fatal(err)
        }

        log.Print("OK")
}

2. What did you expect to see? (Required)

MariaDB(10.3.29) produces: OK

3. What did you see instead (Required)

TiDB(v5.1.0) produces ：Error 1046: No database selected

4. What is your TiDB version? (Required)

Release Version: v5.1.0
Edition: Community
Git Commit Hash: 8acd5c8
Git Branch: release-5.1
UTC Build Time: 2021-07-28 04:40:26
GoVersion: go1.16.5
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[AilinKid]

@morgo do you reckon that this may have something linked with multi statement switch?

[morgo]

@morgo do you reckon that this may have something linked with multi statement switch?

I don't think it is. In this case the user has explictly opted in to multi-statement, so the switch is non-applicable.

It looks like a different bug, where the use db command probably only applies to the next statement, and not the current group of statements. I would still classify it as a protocol bug though, and I can take a look.

[morgo]

I can confirm this bug still exists in 3.0 and 4.0, so it has been present for some time.

[ti-srebot]

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