[PostgreSQL] - explain SQL分析介绍

一、图形化在线分析工具

https://explain.dalibo.com/

二、执行分析语句

EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS, FORMAT JSON)
select * from ...

复制

生成分析JSON之后，填入图形化分析页面，进行分析。

三、分析样例

1、走索引 - Index Scan Node

表示先走二级索引，再走一级索引找到数据

finds relevant records based on an Index. Index Scans perform 2 read operations: one to read the index and another to read the actual value from the table.

2、顺序扫描 - Seq Scan Node

finds relevant records by sequentially scanning the input record set. When reading from a table, Seq Scans (unlike Index Scans) perform a single read operation (only the table is read).

3、缓存 - shared

• Hit - 命中
• Read - 读磁盘
• Dirtied - 脏页(脏页包含在Hit中)

4、循环匹配 - Nested Loop

join数据，将两个结果集进行拼接

merges two record sets by looping through every record in the first set and trying to find a match in the second set. All matching records are returned.

遍历模式

索引模式

5、聚合 - Aggregate

group by 操作

groups records together based on a GROUP BY or aggregate function (like sum()).

6、排序 - Sort

order by 操作

sorts a record set based on the specified sort key.

7、数量限制 - limit

returns a specified number of rows from a record set.

8、with as 临时表 - Common table expressions - CTE

performs a sequential scan of Common Table Expression (CTE) query results. Note that results of a CTE are materialized (calculated and temporarily stored). 对公共表表达式（CTE）查询结果执行顺序扫描。注意，CTE的结果是具体化的（计算并临时存储）。

9、其他操作

• WindowAgg
• Subquery Scan

10、其他join模式图解

• Hash Join

• Merge Join