举个例子: 表b为驱动表,使用 LooseScan 策略按照字段r1分组后和表a做 JOIN。 代码语言:javascript 代码运行次数:0 运行 AI代码解释 localhost:ytt>desc select*from t1 a where r1in(select r1 from t2 b)\G***1.row***id:1select_type:SIMPLEtable:bpartitions:NULLtype:indexpossible_keys:idx_r1key:i...
Not exists:MySQL能够对查询进行LEFT JOIN优化,发现1个匹配LEFT JOIN标准的行后,不再为前面的的行组合在该表内检查更多的行。range checked for each record(index map: #):MySQL没有发现好的可以使用的索引,但发现如果来自前面的表的列值已知,可能部分索引可以使用。Using filesort:当Query中包含 order by 操作...
Avoiding time consuming logic when using flatmapIterable I have a task to split a word into characters and then transfer each to another word. I write some test code, use toCharArray to get char array in the flatMapIterable section, but if the target string... ...
ColumnMapKeyPrune,ConstantCalculator,GroupByConstParallelSetter,LimitOptimizer,LoadTypeCastInserter,MergeFilter,MergeForEach,PartitionFilterOptimizer,PredicatePushdownOptimizer,PushDownForEachFlatten,PushUpFilter,SplitFilter,Stream
HashMap可以看成做数组和链表结合组成的复合结构,数组被分为一个个桶(bucket),通过哈希值决定了键值对在这个数组的寻址;哈希值相同的键值对,则以链表形式存储,如果链表大小超过阈值(TREEIFY_THRESHOLD,默认 8),链表就会被改造成树形结构(红黑树)。转化成红黑树这一过程叫做树形化 树形化还是扩容? 根据哈希表中元素...
一. 语法 explain < table_name > 例如: explain select * from t3 where id=3952602; 二. explain输出解释 +---+---+---+---+---+---+---+---+---+---+ | id | select_type | table | type| possible_keys| key| key_len | ref| rows | ...
range checked for each record (index map: #) mysql没找到合适的可用的索引。取代的办法是,对于前一个表的每一个行连接,它会做一个检验以决定该使用哪个索引(如果有的话),并且使用这个索引来从表里取得记录。这个过程不会很快,但总比没有任何索引时做表连接来得快。
I used a garden to create this color map. And that garden I can actually still change here almost a day after, but at the time I post this link nothing had grown... On a more sad note the other garden which I just became so fond of when I saw it earlier today on another ...
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Range checked for each record (index map: N) MySQL没有找到合适的索引去使用,但是去检查是否可以使用range或index_merge来检索行时,会出现此提示。index map N索引的编号从1开始,按照与表的SHOW INDEX所示相同的顺序。 索引映射值N是指示哪些索引是候选的位掩码值。 例如0x19(二进制11001)的值意味着将考虑索...