unexpect data truncate error when prepare plan cache enabled

[lysu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

enable prepare plan cache

mysql> create table t(data mediumblob);
Query OK, 0 rows affected (0.01 sec)

mysql> prepare stmt1 from 'insert t (data) values (?)';
Query OK, 0 rows affected (0.00 sec)

mysql> set @a = 'a';
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt1 using @a;
Query OK, 1 row affected (0.01 sec)

mysql> set @b = 'aaaaaaaaaaaaaaaaaa';
Query OK, 0 rows affected (0.00 sec)

mysql> execute stmt1 using @b;

2. What did you expect to see?

2 insert ok

3. What did you see instead?

ERROR 1406 (22001): Data too long for column 'data' at row 1
mysql> 

github.com/pingcap/errors.AddStack
	/home/robi/Code/go/pkg/mod/github.com/pingcap/errors@v0.11.0/errors.go:174
github.com/pingcap/parser/terror.(*Error).GenWithStack
	/home/robi/Code/go/pkg/mod/github.com/cloud-pi/parser@v0.0.0-20181211095502-1e4340922693/terror/terror.go:224
github.com/pingcap/tidb/types.ProduceStrWithSpecifiedTp
	/home/robi/Code/go/src/github.com/pingcap/tidb/types/datum.go:839
github.com/pingcap/tidb/types.(*Datum).convertToString
	/home/robi/Code/go/src/github.com/pingcap/tidb/types/datum.go:807
github.com/pingcap/tidb/types.(*Datum).ConvertTo
	/home/robi/Code/go/src/github.com/pingcap/tidb/types/datum.go:686
github.com/pingcap/tidb/expression.(*Constant).Eval
	/home/robi/Code/go/src/github.com/pingcap/tidb/expression/constant.go:102
github.com/pingcap/tidb/executor.(*InsertValues).evalRow
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert_common.go:247
github.com/pingcap/tidb/executor.(*InsertValues).insertRows
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert_common.go:189
github.com/pingcap/tidb/executor.(*InsertExec).Next
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert.go:142
github.com/pingcap/tidb/executor.(*ExecStmt).handleNoDelayExecutor
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/adapter.go:288


[types:1406]Data too long for column 'data' at row 1
github.com/pingcap/errors.AddStack
	/home/robi/Code/go/pkg/mod/github.com/pingcap/errors@v0.11.0/errors.go:174
github.com/pingcap/parser/terror.(*Error).GenWithStack
	/home/robi/Code/go/pkg/mod/github.com/cloud-pi/parser@v0.0.0-20181211095502-1e4340922693/terror/terror.go:224
github.com/pingcap/tidb/executor.resetErrDataTooLong
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/write.go:171
github.com/pingcap/tidb/executor.(*InsertValues).handleErr
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert_common.go:215
github.com/pingcap/tidb/executor.(*InsertValues).evalRow
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert_common.go:248
github.com/pingcap/tidb/executor.(*InsertValues).insertRows
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert_common.go:189
github.com/pingcap/tidb/executor.(*InsertExec).Next
	/home/robi/Code/go/src/github.com/pingcap/tidb/executor/insert.go:142
github.com/pingcap/tidb/executor.(*ExecStmt).handleNoDelayExecutor

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: None
Git Commit Hash: None
Git Branch: None
UTC Build Time: None
GoVersion: None
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false 

[lysu]

The question is:

In first execution, we rewrite ParamMarkerExpr to Constant with DeferredExpr, using current data length, "a" ---> 1 as tp.Flen

tidb/types/field_type.go

Line 176 in 18b33fb

tp.Flen = len(x)

then next execution,

tidb/expression/constant.go

Line 102 in 18b33fb

val, err := dt.ConvertTo(sf.GetCtx().GetSessionVars().StmtCtx, c.RetType)

, will check last Flen=1 instead of real mediumtext len.

[lysu]

@dbjoa how do you think about this ? 😆

[dbjoa]

@lysu I'm going to handle the issue.

[shenli]

@dbjoa Cool!