executor: TopN result in a wrong order

[zhongzc]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

CI task integration-copr-test failed
data: https://github.com/tikv/copr-test/blob/master/push-down-test/prepare/0_data.sql

2. What did you expect to see?

Test pass

3. What did you see instead?

[2020-04-08T13:54:16.385Z] Statement: #507 -  SELECT COALESCE( `col_tinyint_unsigned`, ( COALESCE( ( LEAST( `col_double_unsigned_key`, 4556, ( '2000-01-09' >= ( STRCMP( `col_time_key`, `col_smallint_unsigned_key` ) ) ) ) ), ( ( STRCMP( ( `col_float_unsigned` IS NOT TRUE ), `col_char_255_key` ) ) > ( 'nmo' != -15482 ) ), '1990-06-16 17:22:56.005534' ) ), '2007-07-03', `col_float`, ( `col_float` BETWEEN 0 AND ( ( GREATEST( ( ( `col_set` <=> '2027-11-11' ) <> ( ( ( COALESCE( 'moxqnhbnkyxksjwoaajpbxxobggqewsbvtlqqjkkakmuskosyzsuhdlvfrhobgixtbeqjisgazsdqtccshcxarzvuxsjteeyxmzfxpu', 'p' ) ) IS NULL ) < ( STRCMP( ( `col_int_key` >= 'oxqnhbnkyxksjwoaajpbxxobggqewsbvtlqqjkkakmuskosyzsuhdlvfrhobgixtbeqjisgazsdqtccshcxarzvuxsjteeyxmzfxpuenwuwdczcyaumvpzhxuzftshcwrycsohwtdkrljskkbbbpvsxhzmlpqxozpcrpvqevuvdosgxtlunuhjyomjbucywsqgvbwromzotrdldqfpqvjaxfyhndbzyuuupvchaxe' ), ( -24369 != 'xqnhbnkyxksjwoaajpbxxobggqewsbvtlqqjkkakmuskosyzsuhdlvfrhobgixtbeqjisgazsdqtccshcxarzvuxsjteeyxmzfxpuenwuwdczcyaumvpzhxuzftshcwrycsohwtdkrljskkbbbpvsxhzmlpqxozpcrpvqevuvdosgxtlunuhjyomjbucywsqgvbwromz' ) ) ) ) ), '20:14:46.019164' ) ) <> 'j' ) ) ) AS field1, ISNULL( ( ( ( STRCMP( ( `col_year` IS FALSE ), 7334674943126274048 ) ) IN ( -607985949695016960, `col_varbinary_32`, '2023-03-02 19:05:21.008216', ( 0 < 0 ), `col_set_key` ) ) NOT BETWEEN ( ( 51 IS NOT NULL ) NOT IN ( NULL, ( ISNULL( ( `col_varbinary_32_key` IS UNKNOWN ) ) ) ) ) AND `col_binary_8_key` ) ) AS field2, LEAST( `col_varbinary_32_key`, `col_text_key` ) AS field3 FROM `table1000_int_autoinc` WHERE ( 0 <> `col_decimal_unsigned_key` ) IN ( ( COALESCE( ( `col_double_unsigned` = `col_char_2` ), `col_datetime_key` ) ), ( INTERVAL( ( COALESCE( '04:24:43.033324' ) ), `col_text` ) ) ) ORDER BY field1, field2, field3 LIMIT 7 /* QNO 509 CON_ID 196 */ ;
[2020-04-08T13:54:16.385Z] NoPushDown Output: 
[2020-04-08T13:54:16.385Z] field1	field2	field3
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	0
[2020-04-08T13:54:16.385Z] 0	0	1622605824
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	199229440
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 
[2020-04-08T13:54:16.385Z] 
[2020-04-08T13:54:16.385Z] WithPushDown Output: 
[2020-04-08T13:54:16.385Z] field1	field2	field3
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL
[2020-04-08T13:54:16.385Z] 0	0	NULL

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

master

[zhongzc]

[2020-04-17T07:12:09.261Z] Test case: sql/randgen-topn/2_logical_1.sql
[2020-04-17T07:12:09.261Z] Statement: #984 -  SELECT ( NULL && ( NOT '1991-12-15' ) ) ^ ( `col_timestamp` AND ( `col_char_2` ^ `col_tinyint_key` ) ) AS field1, '11:54:15.059281' << ( ~ `col_varchar_64_key` ) AS field2 FROM `table20_int_autoinc` WHERE NOT `col_decimal` ORDER BY field1, field2 LIMIT 5 /* QNO 989 CON_ID 96 */ ;
[2020-04-17T07:12:09.261Z] NoPushDown Output: 
[2020-04-17T07:12:09.261Z] field1	field2
[2020-04-17T07:12:09.261Z] NULL	0
[2020-04-17T07:12:09.261Z] NULL	0
[2020-04-17T07:12:09.261Z] NULL	0
[2020-04-17T07:12:09.261Z] 0	NULL
[2020-04-17T07:12:09.261Z] 0	NULL
[2020-04-17T07:12:09.261Z] 
[2020-04-17T07:12:09.261Z] 
[2020-04-17T07:12:09.261Z] WithPushDown Output: 
[2020-04-17T07:12:09.261Z] field1	field2
[2020-04-17T07:12:09.261Z] NULL	0
[2020-04-17T07:12:09.261Z] 0	NULL
[2020-04-17T07:12:09.261Z] 0	NULL
[2020-04-17T07:12:09.261Z] 0	0
[2020-04-17T07:12:09.261Z] 0	0

[Reminiscent]

Can be fixed by #16542