SQL Self-Diagnosis

Performance problems may occur when you run the INSERT/UPDATE/DELETE/SELECT/MERGE INTO or CREATE TABLE AS statement. The product supports automatic performance diagnosis and saves related diagnosis information to the Real-time Top SQL. When enable_resource_track is set to on, the diagnosis information is dumped to the Historical Top SQL. You can query the warning field in the views GS_WLM_SESSION_STATISTICS, GS_WLM_SESSION_HISTORY, GS_WLM_SESSION_INFO in Developer Guide to obtain the corresponding performance diagnosis information for performance optimization.

Alarms that can trigger SQL self-diagnosis depend on the settings of resource_track_level. When resource_track_level is set to query, you can diagnose alarms such as uncollected multi-column/single-column statistics, partitions not pruned, and failure of pushing down SQL statements. When resource_track_level is set to perf or operator, all alarms can be diagnosed.

Whether a SQL plan will be diagnosed depends on the settings of resource_track_cost. A SQL plan will be diagnosed only if its execution cost is greater than resource_track_cost. You can use the EXPLAIN keyword to check the plan execution cost.

When EXPLAIN PERFORMANCE or EXPLAIN VERBOSE is executed, SQL self-diagnosis information, except that without multi-column statistics, will be generated. For details, see SQL Execution Plan.

Alarms

Currently, the following performance alarms will be reported:

Statistics of a single column or multiple columns are not collected.
If statistics of a single column or multiple columns are not collected, an alarm is reported. To handle this alarm, you are advised to perform ANALYZE on related tables. For details, see Updating Statistics and Optimizing Statistics.

If no statistics are collected for the OBS foreign table and HDFS foreign table in the query statement, an alarm indicating that statistics are not collected will be reported. Because the ANALYZE performance of the OBS foreign table and HDFS foreign table is poor, you are not advised to perform ANALYZE on these tables. Instead, you are advised to use the ALTER FOREIGN TABLE syntax to modify the totalrows attribute of the foreign table to correct the estimated number of rows.

Example alarms:

The statistics about a table are not collected.
```
Statistic Not Collect
        schema_test.t1
```
The statistics about a single column are not collected.
```
Statistic Not Collect
        schema_test.t2(c1)
```
The statistics about multiple columns are not collected.
```
Statistic Not Collect
        schema_test.t3((c1,c2))
```
The statistics about a single column and multiple columns are not collected.
```
Statistic Not Collect
        schema_test.t4(c1)
        schema_test.t5((c1,c2))
```

Partitions are not pruned (supported by 8.1.2 and later versions).
When a partitioned table is queried, the partition is pruned based on the constraints on the partition key to improve the query performance. However, the partition table may not be pruned due to improper constraints, deteriorating the query performance. For details, see Case: Rewriting SQL Statements and Eliminating Prune Interference.
SQL statements are not pushed down.
The cause details are displayed in the alarms. For details, see Optimizing Statement Pushdown.
The potential causes for the pushdown failure are as follows:
- Caused by functions
  The function name is displayed in the diagnosis information. Function pushdown is determined by the shippable attribute of the function. For details, see the CREATE FUNCTION syntax.
- Caused by syntax
  The diagnosis information displays the syntax that causes the pushdown failure. For example, if the statement contains the With Recursive, Distinct On, or row expression and the return value is of the record type, an alarm is reported, indicating that the syntax does not support pushdown.
Example alarms:
```
SQL is not plan-shipping 
        "enable_stream_operator" is off

SQL is not plan-shipping 
        "Distinct On" can not be shipped

SQL is not plan-shipping
        "v_test_unshipping_log" is VIEW that will be treated as Record type can't be shipped
```

In a hash join, the larger table is used as the inner table.
An alarm will be reported if the number of rows in the inner table reaches or exceeds 10 times of that in the foreign table, more than 100,000 inner-table rows are processed on each DN in average, and data has been flushed to disks. You can view the query_plan column in GS_WLM_SESSION_HISTORY to check whether hash joins are used. In this scenario, you need to adjust the sequence of the HashJoin internal and foreign tables. For details, see Join Order Hints.

Example alarm:
```
Execute diagnostic information
PlanNode[7] Large Table is INNER in HashJoin "Vector Hash Aggregate"
```
In the preceding command, 7 indicates the operator whose ID is 7 in the query_plan column.

nestloop is used in a large-table equivalent join.
An alarm will be reported if nested loop is used in an equivalent join where more than 100,000 larger-table rows are processed on each DN in average. You can view the query_plan column of GS_WLM_SESSION_HISTORY to check whether nested loop is used. In this scenario, you need to adjust the table join mode and disable the NestLoop join mode between the current internal and foreign tables. For details, see Join Operation Hints.

Example alarm:
```
Execute diagnostic information
        PlanNode[5] Large Table with Equal-Condition use Nestloop"Nested Loop"
```

A large table is broadcasted.
An alarm will be reported if more than 100 thousand of rows are broadcasted on each DN in average. In this scenario, the broadcast operation of the BroadCast lower-layer operator needs to be disabled. For details, see Stream Operation Hints.

Example alarm:
```
Execute diagnostic information
        PlanNode[5] Large Table in Broadcast "Streaming(type: BROADCAST dop: 1/2)"
```

Data skew occurs.
An alarm will be reported if the number of rows processed on any DN exceeds 100 thousand, and the number of rows processed on a DN reaches or exceeds 10 times of that processed on another DN. Generally, this alarm is generated due to storage layer skew or computing layer skew. For details, see Optimizing Data Skew.

Example alarm:
```
Execute diagnostic information
       PlanNode[6] DataSkew:"Seq Scan", min_dn_tuples:0, max_dn_tuples:524288
```

The index is improper.
During base table scanning, an alarm is reported if the following conditions are met:
- For row-store tables:
  - When the index scanning is used, the ratio of the number of output lines to the number of scanned lines is greater than 1/1000 and the number of output lines is greater than 10,000.
  - When sequential scanning is used, the number of output lines to the number of scanned lines is less than 1/1000, the number of output lines is less than or equal to 10,000, and the number of scanned lines is greater than 10,000.
- For column-store tables:
  - When the index scanning is used, the ratio of the number of output lines to the number of scanned lines is greater than 1/10000 and the number of output lines is greater than 100.
  - When sequential scanning is used, the number of output lines to the number of scanned lines is less than 1/10,000, the number of output lines is less than or equal to 100, and the number of scanned lines is greater than 10,000.
For details, see Optimizing Operators. You can also refer to Case: Creating an Appropriate Index and Case: Setting Partial Cluster Keys.

Example alarms:
```
Execute diagnostic information
        PlanNode[4] Indexscan is not properly used:"Index Only Scan", output:524288, filtered:0, rate:1.00000
        PlanNode[5] Indexscan is ought to be used:"Seq Scan", output:1, filtered:524288, rate:0.00000
```
The diagnosis result is only a suggestion for the current SQL statement. You are advised to create an index only for frequently used filter criteria.

Estimation is inaccurate.
An alarm will be reported if the maximum number or the estimated maximum number of rows processed on a DN is over 100,000, and the larger number reaches or exceeds 10 times of the smaller one. In this scenario, you can refer to Rows Hints to correct the estimation on the number of rows, so that the optimizer can re-design the execution plan based on the correct number.

Example alarm:
```
Execute diagnostic information
        PlanNode[5] Inaccurate Estimation-Rows: "Hash Join" A-Rows:0, E-Rows:52488
```

Restrictions

An alarm contains a maximum of 2048 characters. If the length of an alarm exceeds this value (for example, a large number of long table names and column names are displayed in the alarm when their statistics are not collected), a warning instead of an alarm will be reported.
```
WARNING, "Planner issue report is truncated, the rest of planner issues will be skipped"
```
If a query statement contains the Limit operator, alarms of operators lower than Limit will not be reported.
For alarms about data skew and inaccurate estimation, only alarms on the lower-layer nodes in a plan tree will be reported. This is because the same alarms on the upper-level nodes may be triggered by problems on the lower-layer nodes. For example, if data skew occurs on the Scan node, data skew may also occur in operators (for example, Hashagg) at the upper layer.