P1-[4.0 bug hunting]-[SQL Plan Management]-Incorrect result or run-time error after changing column type

[sre-bot]

Bug Hunter issue tidb-challenge-program/bug-hunting-issue#10

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Consider the following statements:

CREATE TABLE t0(c0 INT, c1 DECIMAL UNIQUE);
CREATE TABLE t1(c0 INT);
INSERT INTO t0(c1) VALUES (0), (1);
INSERT INTO t1(c0) VALUES (0);
ALTER TABLE t0 MODIFY c1 INT;
SELECT t1.c0 FROM t1, t0 WHERE t0.c1; -- expected: {0}, actual: {0, 0}

Unexpectedly, the SELECT fetches two rows. I would expect only a single one to be fetched. Furthermore, when dropping the UNIQUE constraint, the query fails with an error:

mysql> CREATE TABLE t0(c0 INT, c1 DECIMAL);
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t1(c0 INT);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t0(c1) VALUES (0), (1);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> INSERT INTO t1(c0) VALUES (0);
Query OK, 1 row affected (0.00 sec)

mysql> ALTER TABLE t0 MODIFY c1 INT;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT t1.c0 FROM t1, t0 WHERE t0.c1; -- expected: {0}, actual: {1}
ERROR 1105 (HY000): runtime error: index out of range [7] with length 7

Environment:

| Release Version: v4.0.0-beta.2-231-gc66320c46
Git Commit Hash: c66320c46456c0d5b23b3b0403be6b9f8227d6d8
Git Branch: master
UTC Build Time: 2020-04-14 11:12:29
GoVersion: go1.13.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

An incorrect query result seems to qualify as a P0 bug, according to https://github.com/pingcap/community/blob/master/bug-hunting-programs/bug-hunting-program.md.

[eurekaka]

Looks like we cannot support converting a decimal type column to int type without modifying the underlying data, otherwise the decoder would not work as expected.

runtime.goPanicIndex
	/usr/local/go/src/runtime/panic.go:75
encoding/binary.littleEndian.Uint64
	/usr/local/go/src/encoding/binary/binary.go:76
github.com/pingcap/tidb/util/rowcodec.decodeInt
	/root/go/src/github.com/pingcap/tidb/util/rowcodec/common.go:119
github.com/pingcap/tidb/util/rowcodec.(*ChunkDecoder).decodeColToChunk
	/root/go/src/github.com/pingcap/tidb/util/rowcodec/decoder.go:274
github.com/pingcap/tidb/util/rowcodec.(*ChunkDecoder).DecodeToChunk
	/root/go/src/github.com/pingcap/tidb/util/rowcodec/decoder.go:218
github.com/pingcap/tidb/store/mockstore/unistore/cophandler.(*closureExecutor).tableScanProcessCore
	/root/go/src/github.com/pingcap/tidb/store/mockstore/unistore/cophandler/closure_exec.go:590
github.com/pingcap/tidb/store/mockstore/unistore/cophandler.(*tableScanProcessor).Process
	/root/go/src/github.com/pingcap/tidb/store/mockstore/unistore/cophandler/closure_exec.go:511
github.com/ngaut/unistore/tikv/dbreader.(*DBReader).Scan
	/root/go/pkg/mod/github.com/ngaut/unistore@v0.0.0-20200826054345-a70ab8a1f698/tikv/dbreader/db_reader.go:209
github.com/pingcap/tidb/store/mockstore/unistore/cophandler.(*closureExecutor).execute
	/root/go/src/github.com/pingcap/tidb/store/mockstore/unistore/cophandler/closure_exec.go:385
github.com/pingcap/tidb/store/mockstore/unistore/cophandler.handleCopDAGRequest
	/root/go/src/github.com/pingcap/tidb/store/mockstore/unistore/cophandler/cop_handler.go:79
github.com/pingcap/tidb/store/mockstore/unistore/cophandler.HandleCopRequest
	/root/go/src/github.com/pingcap/tidb/store/mockstore/unistore/cophandler/cop_handler.go:47

@zimulala PTAL

[eurekaka]

A minimal repro:

CREATE TABLE t0(c0 INT, c1 DECIMAL);
INSERT INTO t0(c1) VALUES (0), (1);
ALTER TABLE t0 MODIFY c1 INT;
select c1 from t0;

[bb7133]

Changing DECIMAL to INT is actually not supported. The bug is about a missing error check.

[bb7133]

It is closed by #19645