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system-tables: update CLUSTER_PROCESSLIST and CLUSTER_SLOW_QUERY (#2813) #2900

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Merged
yikeke merged 3 commits into from
Jun 16, 2020
Merged

system-tables: update CLUSTER_PROCESSLIST and CLUSTER_SLOW_QUERY (#2813) #2900

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90ee00b
cherry pick #2813 to release-4.0
ran-huang Jun 16, 2020
b897a32
Merge branch 'release-4.0' into release-4.0-0ebf1c55e8bd
ti-srebot Jun 16, 2020
4e51296
to trigger ci again
yikeke Jun 16, 2020
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53 changes: 53 additions & 0 deletions system-tables/system-table-information-schema.md
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Original file line number Diff line number Diff line change
Expand Up @@ -322,6 +322,28 @@ DEFAULT_CHARACTER_SET_NAME: utf8mb4
4 rows in set (0.00 sec)
```

## CLUSTER_PROCESSLIST

`CLUSTER_PROCESSLIST` is the cluster system table corresponding to `PROCESSLIST`. It is used to query the `PROCESSLIST` information of all TiDB nodes in the cluster. The table schema of `CLUSTER_PROCESSLIST` has one more column than `PROCESSLIST`, the `INSTANCE` column, which stores the address of the TiDB node this row of data is from.

{{< copyable "sql" >}}

```sql
SELECT * FROM information_schema.cluster_processlist;
```

```
+-----------------+-----+------+----------+------+---------+------+------------+------------------------------------------------------+-----+----------------------------------------+
| INSTANCE | ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO | MEM | TxnStart |
+-----------------+-----+------+----------+------+---------+------+------------+------------------------------------------------------+-----+----------------------------------------+
| 10.0.1.22:10080 | 150 | u1 | 10.0.1.1 | test | Query | 0 | autocommit | select count(*) from usertable | 372 | 05-28 03:54:21.230(416976223923077223) |
| 10.0.1.22:10080 | 138 | root | 10.0.1.1 | test | Query | 0 | autocommit | SELECT * FROM information_schema.cluster_processlist | 0 | 05-28 03:54:21.230(416976223923077220) |
| 10.0.1.22:10080 | 151 | u1 | 10.0.1.1 | test | Query | 0 | autocommit | select count(*) from usertable | 372 | 05-28 03:54:21.230(416976223923077224) |
| 10.0.1.21:10080 | 15 | u2 | 10.0.1.1 | test | Query | 0 | autocommit | select max(field0) from usertable | 496 | 05-28 03:54:21.230(416976223923077222) |
| 10.0.1.21:10080 | 14 | u2 | 10.0.1.1 | test | Query | 0 | autocommit | select max(field0) from usertable | 496 | 05-28 03:54:21.230(416976223923077225) |
+-----------------+-----+------+----------+------+---------+------+------------+------------------------------------------------------+-----+----------------------------------------+
```

### SESSION_VARIABLES table

The `SESSION_VARIABLES` table provides information about session variables. The table data is similar to the result of the `SHOW SESSION VARIABLES` statement:
Expand Down Expand Up @@ -406,6 +428,7 @@ desc information_schema.slow_query;
| Cop_wait_addr | varchar(64) | YES | | <null> | |
| Mem_max | bigint(20) unsigned | YES | | <null> | |
| Succ | tinyint(1) unsigned | YES | | <null> | |
| Plan_from_cache | tinyint(1) | YES | | <null> | |
| Plan | longblob unsigned | YES | | <null> | |
| Plan_digest | varchar(128) | YES | | <null> | |
| Prev_stmt | longblob unsigned | YES | | <null> | |
Expand Down Expand Up @@ -470,13 +493,43 @@ desc information_schema.cluster_slow_query;
| Cop_wait_addr | varchar(64) | YES | | <null> | |
| Mem_max | bigint(20) unsigned | YES | | <null> | |
| Succ | tinyint(1) unsigned | YES | | <null> | |
| Plan_from_cache | tinyint(1) | YES | | <null> | |
| Plan | longblob unsigned | YES | | <null> | |
| Plan_digest | varchar(128) | YES | | <null> | |
| Prev_stmt | longblob unsigned | YES | | <null> | |
| Query | longblob unsigned | YES | | <null> | |
+---------------------------+---------------------+------+-----+---------+-------+
```

When the cluster system table is queried, TiDB does not obtain data from all nodes, but pushes down the related calculation to other nodes. The execution plan is as follows:

{{< copyable "sql" >}}

```sql
desc select count(*) from information_schema.cluster_slow_query where user = 'u1';
```

```
+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+
| StreamAgg_20 | 1.00 | root | | funcs:count(Column#53)->Column#51 |
| └─TableReader_21 | 1.00 | root | | data:StreamAgg_9 |
| └─StreamAgg_9 | 1.00 | cop[tidb] | | funcs:count(1)->Column#53 |
| └─Selection_19 | 10.00 | cop[tidb] | | eq(information_schema.cluster_slow_query.user, "u1") |
| └─TableFullScan_18 | 10000.00 | cop[tidb] | table:CLUSTER_SLOW_QUERY | keep order:false, stats:pseudo |
+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+
```

In the above execution plan, the `user = u1` condition is pushed down to other (`cop`) TiDB nodes, and the aggregate operator is also pushed down (the `StreamAgg` operator in the graph).

Currently, because statistics of the system tables are not collected, sometimes some aggregation operators cannot be pushed down, which results in slow execution. In this case, you can manually specify the SQL HINT to push down the aggregation operators. For example:

{{< copyable "sql" >}}

```sql
select /*+ AGG_TO_COP() */ count(*) from information_schema.cluster_slow_query group by user;

### STATISTICS table

The `STATISTICS` table provides information about table indexes:
Expand Down