E:using index:查询时不需要回表查询,直接通过索引就可以获取查询的数据。 F:using join buffer(block nested loop),using join buffer(batched key accss):5.6.x之后的版本优化关联查询的BNL,BKA特性。主要是减少内表的循环数量以及比较顺序地扫描查询。 G:using sort_union,using_union,using intersect,using so...
日常工作中,我们有时会通过日志记录下耗时较长的SQL语句,但是光找出这些SQL语句并不意味着完事了,常常需要借助EXPLAIN来查看SQL语句的执行计划,查看SQL语句是否用上了索引,是否进行了全表扫描,这都可以通过EXPLAIN命令得到。 概述 EXPLAIN:为SELECT语句中使用到的每个表返回一条信息。它按照MySQL在处理语句时读取它们的顺...
2.11.2 Not exists 2.11.3 range checked for each record 2.11.4 using filesort (MySQL中无法利用索引完成的排序操作称为“文件排序”) 2.11.5 using index (代表索引覆盖,就是查询的列正好在索引中,不用回物理行查询数据) 2.11.6 using temporary (表示MySQL需要使用临时表来存储结果集,常见于排序和分组查询...
Using index for skip scan (JSON属性: using_index_for_skip_scan) 表示使用了Skip Scan访问方法。请参见Skip Scan范围访问方法。 Using join buffer (Block Nested Loop), Using join buffer (Batched Key Access), Using join buffer (hash join) (JSON属性: using_join_buffer) 早期连接的表以部分方式读...
UNCACHEABLE SUBQUERY A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query UNCACHEABLE UNION The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY) 常用类型示例: SIMPLE:简单的查询 mysql>...
NULL|NULL|NULL|NULL|4997|100.00|Usingjoinbuffer (Block Nested Loop)|+---+---+---+---+---+---+---+---+---+---+---+---+ id 每个表都会对应一条记录 查询语句中每出现一个SELECT关键字, MySQL会为它分配一个唯一的id值。 这个id值就是EXPLAIN语句的第一个列,对于连接查询来说, 一...
foreach x in array $2 loop res := array_append(res, $1||'_'||x); end loop; return res; end; $$ language plpgsql strict immutable; postgres=# select gen_newarr(123,array[1,2,3,4]); -[ RECORD 1 ]--- gen_newarr | {123_1,123_2,123...
where 'i' is the loop variable The 'do-while' loop can be implemented (in C) as: inti=5; do { printf("%d",i); i--; }while(i>=0); where 'i' is the loop variable. Answer and Explanation:1 Both for loop and while loop can run multiple statements in successive repetition effic...
DFELoopSubQuery – This is a non-blocking operation that acts as a wrapper for a subquery, allowing it to be run repeatedly for use in loops. DFEMergeChunks – This is a blocking operation that combines chunks from its upstream operator into a single chunk of solutions to pass to its down...
LEAVE explain_loop; END IF; CALL explain_query_for_schema(schema_name, query); END LOOP; CLOSE cur_schema_names; END | DELIMITER ; SET @query = "SELECT name, quantity FROM orders JOIN items ON orders.item_id = items.id"; CALL explain_query_for_all_schemas(@query); ...