Optimizer choose full table scan when there is a better execution path

[mzhang77]

Bug Report

1. Minimal reproduce step (Required)

mysql> show create table d\G
*************************** 1. row ***************************
       Table: d
Create Table: CREATE TABLE `d` (
  `mid` bigint(20) unsigned NOT NULL,
  `object_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `ov` longblob DEFAULT NULL,
  `version` int(11) unsigned NOT NULL,
  PRIMARY KEY (`object_id`) /*T![clustered_index] CLUSTERED */
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_general_ci AUTO_INCREMENT=23924932 /*T![auto_id_cache] AUTO_ID_CACHE=1 */
1 row in set (0.00 sec)
mysql> show create table i\G
*************************** 1. row ***************************
       Table: i
Create Table: CREATE TABLE `i` (
  `object_id` bigint(20) unsigned NOT NULL,
  `lid` bigint(20) DEFAULT NULL,
  `sid` varbinary(767) DEFAULT NULL,
  `cid` bigint(20) DEFAULT NULL,
  `version` int(11) unsigned NOT NULL,
  PRIMARY KEY (`object_id`) /*T![clustered_index] CLUSTERED */,
  KEY `i_1` (`cid`,`sid`,`object_id`),
  KEY `i_2` (`lid`,`object_id`),
  KEY `i_3` (`cid`,`object_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_general_ci
1 row in set (0.00 sec)

mysql> 
explain SELECT
       `d`.*
     FROM
       `i`
       LEFT JOIN `d` ON `i`.`object_id` = `d`.`object_id`
     WHERE
       `i`.`cid` = 249
     ORDER BY
       `i`.`object_id`
     LIMIT 1000 OFFSET 18000;

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

The execution plan should use table i as driving table to access table d

3. What did you see instead (Required)

The problem is captured by slow log, it is not reproducible any more.

The issue with this plan is, there is no need to full table scan d. Optimizer should offset 18000 rows from i, take the next 1000 rows, and join them to table d. So maybe full table scan i is necessary, but full table scan d to do a merge join is unnecessary.

4. What is your TiDB version? (Required)

v7.5.1

[mzhang77]

I'm able to reproduce the problem using this script to generate test data: https://gist.github.com/mzhang77/a7cb08949d8473edbde4a27bde0d3530

[winoros]

Cost for default choice MergeJoin

mysql>  explain format='cost_trace' SELECT  `d`.* FROM   `i`   LEFT JOIN `d` ON `i`.`object_id` = `d`.`object_id` WHERE   `i`.`cid` = 249 ORDER BY   `i`.`object_id` LIMIT 1000 OFFSET 18000;
+-----------------------------------+----------+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+------------------------------------------------------------------------+
| id                                | estRows  | estCost    | costFormula                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | task      | access object                      | operator info                                                          |
+-----------------------------------+----------+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+------------------------------------------------------------------------+
| Projection_12                     | 1000.00  | 1417546.02 | (((((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(30000*filters(0)*tidb_cpu_factor(49.9)))) + ((group(19000*cols(0.01)*tidb_cpu_factor(49.9))) + (group(30000*cols(0.01)*tidb_cpu_factor(49.9)))))) + ((cpu(1000*filters(0.04)*tidb_cpu_factor(49.9)))/5.00) | root      |                                    | test.d.mid, test.d.object_id, test.d.ov, test.d.version                |
| └─Limit_19                        | 1000.00  | 1417146.82 | ((((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(30000*filters(0)*tidb_cpu_factor(49.9)))) + ((group(19000*cols(0.01)*tidb_cpu_factor(49.9))) + (group(30000*cols(0.01)*tidb_cpu_factor(49.9)))))                                                            | root      |                                    | offset:18000, count:1000                                               |
|   └─MergeJoin_67                  | 19000.00 | 1417146.82 | (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(30000*filters(0)*tidb_cpu_factor(49.9)))) + ((group(19000*cols(0.01)*tidb_cpu_factor(49.9))) + (group(30000*cols(0.01)*tidb_cpu_factor(49.9))))                                                              | root      |                                    | left outer join, left key:test.i.object_id, right key:test.d.object_id |
|     ├─TableReader_53(Build)       | 30000.00 | 1024516.39 | ((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                | root      |                                    | data:TableFullScan_52                                                  |
|     │ └─TableFullScan_52          | 30000.00 | 7406957.80 | scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | cop[tikv] | table:d                            | keep order:true                                                        |
|     └─Limit_36(Probe)             | 19000.00 | 368179.43  | ((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)                                                                                                                                                                                                                                                                                                                                                                                                                          | root      |                                    | offset:0, count:19000                                                  |
|       └─IndexReader_49            | 19000.00 | 368179.43  | (((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                                            | root      |                                    | index:Limit_48                                                         |
|         └─Limit_48                | 19000.00 | 4318851.50 | (scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | cop[tikv] |                                    | offset:0, count:19000                                                  |
|           └─IndexRangeScan_47     | 19000.00 | 4318851.50 | scan(19000*logrowsize(48)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | cop[tikv] | table:i, index:i_3(cid, object_id) | range:[249,249], keep order:true                                       |
+-----------------------------------+----------+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+------------------------------------------------------------------------+

Cost for index join choice

mysql>  explain format='cost_trace' SELECT /*+ INL_JOIN(d) */   `d`.* FROM   `i`   LEFT JOIN `d` ON `i`.`object_id` = `d`.`object_id` WHERE   `i`.`cid` = 249 ORDER BY   `i`.`object_id` LIMIT 1
000 
+-------------------------------------+----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| id                                  | estRows  | estCost     | costFormula                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | task      | access object                      | operator info                                                                                                                                    |
+-------------------------------------+----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| Projection_11                       | 1000.00  | 10556981.64 | (((cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(19000*10*tidb_cpu_factor(49.9))) + ((() + (((((scan(1*logrowsize(540)*tikv_scan_factor(40.7))) + (net(1*rowsize(540)*tidb_kv_net_factor(3.96))))/15.00)*19000.00)/6.00) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(19000*0*tidb_cpu_factor(49.9))) + (hashmem(19000*540*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))))/5.00))) + ((cpu(1000*filters(0.04)*tidb_cpu_factor(49.9)))/5.00) | root      |                                    | test.d.mid, test.d.object_id, test.d.ov, test.d.version                                                                                          |
| └─Limit_18                          | 1000.00  | 10556582.44 | ((cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(19000*10*tidb_cpu_factor(49.9))) + ((() + (((((scan(1*logrowsize(540)*tikv_scan_factor(40.7))) + (net(1*rowsize(540)*tidb_kv_net_factor(3.96))))/15.00)*19000.00)/6.00) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(19000*0*tidb_cpu_factor(49.9))) + (hashmem(19000*540*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))))/5.00))                                                            | root      |                                    | offset:18000, count:1000                                                                                                                         |
|   └─IndexJoin_54                    | 19000.00 | 10556582.44 | (cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (cpu(19000*10*tidb_cpu_factor(49.9))) + ((() + (((((scan(1*logrowsize(540)*tikv_scan_factor(40.7))) + (net(1*rowsize(540)*tidb_kv_net_factor(3.96))))/15.00)*19000.00)/6.00) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(19000*0*tidb_cpu_factor(49.9))) + (hashmem(19000*540*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))))/5.00)                                                              | root      |                                    | left outer join, inner:TableReader_51, outer key:test.i.object_id, inner key:test.d.object_id, equal cond:eq(test.i.object_id, test.d.object_id) |
|     ├─Limit_63(Build)               | 19000.00 | 368179.43   | ((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | root      |                                    | offset:0, count:19000                                                                                                                            |
|     │ └─IndexReader_69              | 19000.00 | 368179.43   | (((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | root      |                                    | index:Limit_68                                                                                                                                   |
|     │   └─Limit_68                  | 19000.00 | 4318851.50  | (scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | cop[tikv] |                                    | offset:0, count:19000                                                                                                                            |
|     │     └─IndexRangeScan_44       | 19000.00 | 4318851.50  | scan(19000*logrowsize(48)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | cop[tikv] | table:i, index:i_3(cid, object_id) | range:[249,249], keep order:true                                                                                                                 |
|     └─TableReader_51(Probe)         | 19000.00 | 167.19      | ((scan(1*logrowsize(540)*tikv_scan_factor(40.7))) + (net(1*rowsize(540)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | root      |                                    | data:TableRangeScan_50                                                                                                                           |
|       └─TableRangeScan_50           | 19000.00 | 369.43      | scan(1*logrowsize(540)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | cop[tikv] | table:d                            | range: decided by [test.i.object_id], keep order:false                                                                                           |
+-------------------------------------+----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
9 rows in set (0.00 sec)

Cost for hash join choice

mysql>  explain format='cost_trace' SELECT /*+ HASH_JOIN(d) */  `d`.* FROM   `i`   LEFT JOIN `d` ON `i`.`object_id` = `d`.`object_id` WHERE   `i`.`cid` = 249 ORDER BY   `i`.`object_id` LIMIT 1000 OFFSET 18000;
+-------------------------------------+----------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+-----------------------------------------------------------------+
| id                                  | estRows  | estCost     | costFormula                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | task      | access object                      | operator info                                                   |
+-------------------------------------+----------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+-----------------------------------------------------------------+
| Projection_11                       | 1000.00  | 19508812.18 | (((cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((hashkey(19000*1*tidb_cpu_factor(49.9))) + (hashmem(19000*16*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (((cpu(30000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(30000*1*tidb_cpu_factor(49.9))) + (hashprobe(30000*tidb_cpu_factor(49.9)))))/5.00)) + ((exprCPU(19000*0*tidb_cpu_factor(49.9))) + (orderCPU(19000*log(19000)*tidb_cpu_factor(49.9)))) + (topMem(19000*548*tidb_mem_factor(0.2)))) + ((cpu(1000*filters(0.04)*tidb_cpu_factor(49.9)))/5.00) | root      |                                    | test.d.mid, test.d.object_id, test.d.ov, test.d.version         |
| └─TopN_14                           | 1000.00  | 19508412.98 | ((cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((hashkey(19000*1*tidb_cpu_factor(49.9))) + (hashmem(19000*16*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (((cpu(30000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(30000*1*tidb_cpu_factor(49.9))) + (hashprobe(30000*tidb_cpu_factor(49.9)))))/5.00)) + ((exprCPU(19000*0*tidb_cpu_factor(49.9))) + (orderCPU(19000*log(19000)*tidb_cpu_factor(49.9)))) + (topMem(19000*548*tidb_mem_factor(0.2)))                                                            | root      |                                    | test.i.object_id, offset:18000, count:1000                      |
|   └─HashJoin_30                     | 19000.00 | 3949992.82  | (cpu(10*3*tidb_cpu_factor(49.9))) + (((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)) + (((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00) + ((hashkey(19000*1*tidb_cpu_factor(49.9))) + (hashmem(19000*16*tidb_mem_factor(0.2))) + (hashbuild(19000*tidb_cpu_factor(49.9)))) + (cpu(19000*filters(0)*tidb_cpu_factor(49.9))) + (((cpu(30000*filters(0)*tidb_cpu_factor(49.9))) + ((hashkey(30000*1*tidb_cpu_factor(49.9))) + (hashprobe(30000*tidb_cpu_factor(49.9)))))/5.00)                                                                                                                                                                                                           | root      |                                    | left outer join, equal:[eq(test.i.object_id, test.d.object_id)] |
|     ├─Limit_35(Build)               | 19000.00 | 368179.43   | ((((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | root      |                                    | offset:0, count:19000                                           |
|     │ └─IndexReader_48              | 19000.00 | 368179.43   | (((scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))) + (net(19000*rowsize(16)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | root      |                                    | index:Limit_47                                                  |
|     │   └─Limit_47                  | 19000.00 | 4318851.50  | (scan(19000*logrowsize(48)*tikv_scan_factor(40.7)))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | cop[tikv] |                                    | offset:0, count:19000                                           |
|     │     └─IndexRangeScan_46       | 19000.00 | 4318851.50  | scan(19000*logrowsize(48)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | cop[tikv] | table:i, index:i_3(cid, object_id) | range:[249,249], keep order:true                                |
|     └─TableReader_52(Probe)         | 30000.00 | 1024516.39  | ((scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))) + (net(30000*rowsize(67.00999999999999)*tidb_kv_net_factor(3.96))))/15.00                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | root      |                                    | data:TableFullScan_51                                           |
|       └─TableFullScan_51            | 30000.00 | 7406957.80  | scan(30000*logrowsize(67.00999999999999)*tikv_scan_factor(40.7))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | cop[tikv] | table:d                            | keep order:false                                                |
+-------------------------------------+----------+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------+------------------------------------+-----------------------------------------------------------------+
9 rows in set (0.00 sec)

[mzhang77]

For INL_JOIN, table range scan of d should have estRows of 1000, instead of 19000

[winoros]

For INL_JOIN, table range scan of d should have estRows of 1000, instead of 19000

@mzhang77
Oh, I got your point.

From the explain result, when we push a top-n down to the outer join's outer child. We'll remove the offset and make a full TopN

Maybe this part can be improved. We'll check it later.

[winoros]

@mzhang77
Some explanation about why we remove the offset when pushing it down:
The original TopN is applied after the join. One row from the outer join's outer child might match more than one row from the inner side. So we can only push down a TopN(0, 19000) to the outer child. We don't know which row from the outer side will produce the 1001-th join result.
Due to this reason, we also don't know how many rows the inner side needs output to make the join output the 19000-th join result.

So currently, both sides will use 19000 rows to calculate the cost.

Oh, but I noticed that your join key from the inner side is the primary key, so this can ensure each match of the outer side is exactly one. So the total execution logic can be improved maybe.

If the join key doesn't have the unique property, i'm afraid that the execution logic and the estimation logic will not change.

[mzhang77]

@winoros This is a real case from a paid customer. Actually in their application all join key has unique property.

[winoros]

@winoros This is a real case from a paid customer. Actually in their application all join key has unique property.

i know. i just want to explain the current situation and the possible improvment tidb can do.