pesudo statistics of empty table

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  KEY `a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY HASH( `a` )
PARTITIONS 4;

insert into t values (1), (2), (3);
analyze table t;
desc select * from t;

2. What did you expect to see? (Required)

The estRows of p0 should be 0 and the total estRows should be 3;

3. What did you see instead (Required)

mysql> desc select * from t;
+----------------------------+----------+-----------+-----------------------+--------------------------------+
| id                         | estRows  | task      | access object         | operator info                  |
+----------------------------+----------+-----------+-----------------------+--------------------------------+
| PartitionUnion_10          | 10003.00 | root      |                       |                                |
| ├─TableReader_12           | 10000.00 | root      |                       | data:TableFullScan_11          |
| │ └─TableFullScan_11       | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
| ├─TableReader_16           | 1.00     | root      |                       | data:TableFullScan_15          |
| │ └─TableFullScan_15       | 1.00     | cop[tikv] | table:t, partition:p1 | keep order:false               |
| ├─TableReader_20           | 1.00     | root      |                       | data:TableFullScan_19          |
| │ └─TableFullScan_19       | 1.00     | cop[tikv] | table:t, partition:p2 | keep order:false               |
| └─TableReader_24           | 1.00     | root      |                       | data:TableFullScan_23          |
|   └─TableFullScan_23       | 1.00     | cop[tikv] | table:t, partition:p3 | keep order:false               |
+----------------------------+----------+-----------+-----------------------+--------------------------------+
9 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

mysql> select version();
+----------------------------------------+
| version()                              |
+----------------------------------------+
| 5.7.25-TiDB-v5.2.0-alpha-10-g1aea274cc |
+----------------------------------------+
1 row in set (0.00 sec)

[rebelice]

It's not a bug about the partition table. It has the same behavior as the normal table.

MySQL [test]> create table t(a int, key(a));
Query OK, 0 rows affected (0.010 sec)

MySQL [test]> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.001 sec)

MySQL [test]> analyze table t;
Query OK, 0 rows affected (0.026 sec)

MySQL [test]> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.001 sec)

[yudongusa]

seems similar issue of #27083