此时我们常常用到 explain 这个命令来查看一个这些 SQL 语句的执行计划,查看该 SQL 语句有没有使用上了索引,有没有做全表扫描。 id id 是用来顺序标识整个查询中 SELELCT 语句的,在嵌套查询中 id 越大的语句越先执行。该值可能为 NULL,如果这一行用来说...mysql性能优化------explain详解 1.explain作用 explain语句提供了
unionM,N: The row refers to the union of the rows with id values of M and N. derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. subqueryN: The row refers to the...
* Work with a copy instead of modifying the leader state, since this * function may be called twice */ if (queryDesc->estate->es_jit) InstrJitAgg(&ji, &queryDesc->estate->es_jit->instr); /* If this process has done JIT in parallel workers, merge stats */ if (queryDesc->estate...
the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving several tables can be performedwithout comparing every combination of rows. The set of operations...
摘要: DBA经常会用到的explain来查看SQL语句的执行计划,今天小人斗胆,从MySQL 5.7 Reference Manual中把MySQL EXPLAIN Output Format翻译过来。欢迎拍砖 Explain语句提供了一个select语句执行计划的信息。 Explain为每个用了select语句的表,返回一行信息。它列出了表中的顺序输出,MySQL会读取他们,然后再处理。MySQL解决了所...
解答:运行utlxplan.sql. 建立plan 表 针对特定SQL语句,使用 explain plan set statement_id = 'tst1' into plan_table 运行utlxplp.sql 或 utlxpls.sql察看explain plan EXPLAIN PLAN 是一个很好的分析SQL语句的工具,它甚至可以在不执行SQL的情况下分析语句. 通过分析,我们就可以知道ORACLE是怎么样连接表,使用...
Using the DBMS_XPLAN.DISPLAY_CURSOR Function Alternatively the execution plan for an executed SQL statement can be generated and displayed by using the DBMS_XPLAN.DISPLAY_CURSOR function. The following example shows the plan for the previously executed SQL statement in the session. Figure 4: ...
Example kdb+ AI Explanation Output / This SQL code is a function called `toxml`. It creates an XML representation of a table in the KDB database. Here is a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 /give...
If it is not acceptable for some reason, we'll need to wrap this query into a subquery, but I won't do it in this example for the sake of brevity. To be continued. Share this: Facebook X Leave a Reply
If you want to query execution plans with higher readability, we recommend that you upgrade your Hologres instance to V1.3.4x or later. EXPLAIN Syntax You can execute the following EXPLAIN statement to query the execution plan that is estimated by the QO: EXPLAIN <sql>; Example In this ...