Details
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Bug
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Status: Closed (View Workflow)
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Critical
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Resolution: Fixed
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10.3.10, 10.2(EOL), 10.3(EOL)
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None
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Ubuntu 18.04.1 LTS
Description
select * from days where date < '2018-10-09' order by date desc limit 1
fails if there is an index on date and a record for 2018-10-09 does not exist in the table.
To recreate ....
create table days (date date);
insert into days values ('2018-10-04'), ('2018-10-05');
select * from days where date < '2018-10-09' order by date desc limit 1; # Works as expected
alter table days add index date_index (date);
select * from days where date < '2018-10-05' order by date desc limit 1; # Works as expected
select * from days where date < '2018-10-09' order by date desc limit 1; # FAILS: Empty set (0.001 sec)
Attachments
Activity
Not reproducible on the upstream.
Well, the upstream produces a difference query plan:
MariaDB:
mysql> explain select * from days where date < '2018-10-09' order by date desc limit 1;
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+------+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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| 1 | SIMPLE | days | range | date_index | date_index | 4 | NULL | 1 | Using where; Using index |
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+------+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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1 row in set (0.00 sec)
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Upstream, current FB/mysql-5.6
mysql> explain select * from days force index(date_index) where date < '2018-10-09' order by date desc limit 1;
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+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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| 1 | SIMPLE | days | index | date_index | date_index | 4 | NULL | 2 | Using where; Using index |
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+----+-------------+-------+-------+---------------+------------+---------+------+------+--------------------------+
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1 row in set (0.00 sec)
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... but if I add more rows to the table I can get it to also pick the range plan, and it will produce the correct result.
Investigation:
The range we are scanning is actually NULL < date < '2018-10-09'
Date column is 3 bytes + 1 NULL byte.
'2018-10-09' is encoded as
uint tmp= ltime->year*16*32 + ltime->month*32+ltime->day = 0x0f c5 45
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Inside myrocks::ha_rocksdb::calc_eq_cond_len, myrocks::ha_rocksdb::index_read_map_impl, we have:
(gdb) p slice
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$302 = (const rocksdb::Slice &) @0x7fffd9934210: {data_ = 0x7fff783a7dd0 "", size_ = 8}
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(gdb) x/8cx slice.data_
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0x7fff783a7dd0: 0x00 0x00 0x01 0x07 0x01 0x0f 0xc5 0x49
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(gdb) p *end_key
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$307 = {key = 0x7fff78343c60 "", length = 4, keypart_map = 1, flag = HA_READ_BEFORE_KEY}
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(gdb) x/4x end_key.key
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0x7fff78343c60: 0x00 0x49 0xc5 0x0f
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That is, it thinks that both start and the end keys are '2018-10-09'.
Comparing with the upstream:
(gdb) p slice
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$79 = (const rocksdb::Slice &) @0x7ffff40b41f0: {data_ = 0x7fffcc024bf0 "", size_ = 8}
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(gdb) x/8cx slice.data_
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0x7fffcc024bf0: 0x00 0x00 0x01 0x04 0x01 0x0f 0xc5 0x49
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(gdb) p *end_key
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$80 = {key = 0x7fffcc0d3808 "\001", length = 4, keypart_map = 1, flag = HA_READ_AFTER_KEY}
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(gdb) x/4cx end_key->key
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0x7fffcc0d3808: 0x01 0x00 0x00 0x00
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The SE API doesn't have a direct equivalent of read_range_first/next for doing reverse scans.
Instead, the QUICK_SELECT_DESC does this:
1. Inform the SE about the other end of the range.
2. Call ha_rocksdb->ha_index_read_map(..., find_flag=HA_READ_BEFORE_KEY)
Step #2 is implemented in different ways in MariaDB and in the Upstream.
The upstream' QUICK_SELECT_DESC::get_next does it as follows:
file->set_end_range(&min_range, handler::RANGE_SCAN_DESC);
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min_range here has
{key= SQL "NULL", flag = HA_READ_AFTER_KEY}MariaDB's QUICK_SELECT_DESC::get_next uses a different API call:
result= file->prepare_range_scan((last_range->flag & NO_MIN_RANGE) ? NULL : &start_key, |
(last_range->flag & NO_MAX_RANGE) ? NULL : &end_key); |
(gdb) x/4x start_key.key
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0x7fff78343f48: 0x01 0x00 0x00 0x00 // HA_READ_AFTER_KEY
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(gdb) x/4x end_key.key
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0x7fff78343f50: 0x00 0x49 0xc5 0x0f // HA_READ_BEFORE_KEY
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index_read_map_impl() actually has the code to flip the bounds:
const key_range *cur_end_key= end_key; |
if (find_flag == HA_READ_PREFIX_LAST_OR_PREV) |
cur_end_key= m_start_range;
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added by
commit 4d51009a772fdfffc6cc7ae87422a3b0b19fa06a
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Author: Sergei Petrunia <psergey@askmonty.org> Tue May 8 13:00:26 2018
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Committer: Sergei Petrunia <psergey@askmonty.org> Tue May 8 13:00:26 2018
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But it only handles closed ranges, not open ones
Thanks for the report and test case. Reproducible as described.
Same workflow with InnoDB or MyISAM works all right.