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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N/A
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None
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preview-10.7-
MDEV-26519-json-histograms c548019b
Description
drop table if exists t1; |
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create table t1 (c char(8)) engine=MyISAM; |
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insert into t1 values ('1x'); |
insert into t1 values ('1x'); |
insert into t1 values ('1xx'); |
insert into t1 values ('0xx'); |
insert into t1 select * from t1; |
insert into t1 select * from t1; |
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set histogram_type= SINGLE_PREC_HB; |
analyze table t1 persistent for all; |
analyze
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select c from t1 where c > '1'; |
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set histogram_type= DOUBLE_PREC_HB; |
analyze table t1 persistent for all; |
analyze
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select c from t1 where c > '1'; |
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set histogram_type= JSON_HB; |
analyze table t1 persistent for all; |
analyze
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select c from t1 where c > '1'; |
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# Cleanup
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drop table t1; |
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SINGLE_PREC_HB |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 16 | 16.00 | 74.90 | 75.00 | Using where | |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
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DOUBLE_PREC_HB |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 16 | 16.00 | 75.00 | 75.00 | Using where |
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+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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JSON_HB |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 16 | 16.00 | 33.33 | 75.00 | Using where | |
+------+-------------+-------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+ |
Results from preview-10.7-MDEV-26519-json-histograms c548019b
Attachments
Issue Links
- is caused by
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MDEV-21130 Histograms: use JSON as on-disk format
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- Closed
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Activity
There are some general considerations about observed histogram precision, but let's tackle this particular case, first.
Data distribution:
MariaDB [j4]> select c, count(*) from t1 group by c;
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+------+----------+
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| c | count(*) |
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+------+----------+
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| 0xx | 4 |
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| 1x | 8 |
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| 1xx | 4 |
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+------+----------+
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The histogram seems adequate:
{
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"histogram_hb_v2": [
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{
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"start": "0xx",
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"size": 0.25,
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"ndv": 1
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},
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{
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"start": "1x",
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"size": 0.5,
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"ndv": 1
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},
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{
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"start": "1xx",
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"end": "1xx",
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"size": 0.25,
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"ndv": 1
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}
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]
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}
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.. but things go wrong at the phase where we are computing the selectivity.
Raised priority on procedural reasons