小结: 1.覆盖索引 回表 2. All indexes in PostgreSQL are secondary indexes, meaning that each index is stored separately from the table's main data area (which is called the table's heap in PostgreSQL terminology). This means that in an ordinary index scan, ea…

Covering Indexes in MySQL, PostgreSQL, and MongoDB - Orange Matter https://orangematter.solarwinds.com/2019/02/01/covering-indexes-in-mysql-postgresql-and-mongodb/ Query Optimization - MongoDB Manual https://docs.mongodb.com/manual/core/query-optimiz…

今天给大家介绍两个黑魔法,这都是压箱底的法宝.大家在使用时,一定要弄清他们的适用场景及用法,用好了,就是一把开天斧,用不好那就是画蛇添足.自从看过耗子哥(左耳朵耗子)的博客,都会给对相应专题有兴趣的小伙伴列出几篇拓展文章,我觉得这种方式还是非常不错,所以这篇文章我也会列出几篇扩展的文章,供想更深入思考的小伙伴查阅. 可能有人会认为这两个用法会比较冷门,但是在跨系统调用api的过程中,表的数据量比较大时,sql查询性能太差,会导致接口响应超时,就会对相应的业务产生非常大的影响.系统优化,大家千万不…

可能有小伙伴会问,Covering Indexes到底是什么神器呢?它又是如何来提升性能的呢?接下来我会用最通俗易懂的语言来进行介绍,毕竟不是每个程序猿都要像DBA那样深刻理解数据库,知道如何用以及如何用好神器才是最关键的. Covering Indexes就是一个索引覆盖所有要查询的字段(ps:这句话我挖个坑,文末我来解释). An index that contains all required information to resolve the query is known as a “…

一.聚集索引介绍 1.什么是聚集索引? InnoDB’s clustered indexes actually store a B-Tree index and the rows together in the same structure. 2.为什么一张表只能一个聚集索引? When a table has a clustered index, its rows are actually stored in the index’s leaf pages.The term “clustered…

之前没特别注意,在有些场景下希望使用并行索引扫描的时候,发现parallel_index hint并没有生效,于是抽空看了下文档:The PARALLEL_INDEX hint instructs the optimizer to use the specified number of concurrent servers to parallelize index range scans for partitioned indexes. 但是有很多表即使实时数据量有上千万,但是也无法做到合理的维…

Hints 应该慎用,收集相关表的统计信息,根据执行计划,来改变查询方式 只能在SELECT, UPDATE, INSERT, MERGE, or DELETE 关键字后面,只有insert可以用2个hint(append 跟parallel),其他只能only one—如果sql中使用了hint,则就意味着启用了CBO 1 /*+ gather_plan_statistics*/ 用于在目标sql执行时收集一些额外的统计信息 select /*+ gather_plan_statistics*…

通常来说,索引可以用于提高查询的速度.通过索引,可以快速访问表中的指定数据,避免了表上的扫描.有时候,索引不仅仅能够用于定位表中的数据.某些查询可能只需要访问索引的数据,就能够获取所需要的结果,而不需要再次访问表中的数据.这种访问数据的方法叫做 Index-Only 扫描.要想通过索引直接返回查询的数据,创建的索引需要包含 SELECT 列表中的所有字段:CREATE TABLE t1 (a int, b int, c int); INSERT INTO t1     SELECT val, v…

Table of Contents The ProblemAlgorithmDigressionFirst, some examplesAlgorithm, Step 1 (WHERE "column = const")Algorithm, Step 2Algorithm, Step 2a (one range)Algorithm, Step 2b (GROUP BY)Algorithm, Step 2c (ORDER BY)Algorithm endLimitationsStop a…

487down vote Differences KEY or INDEX refers to a normal non-unique index.  Non-distinct values for the index are allowed, so the index may contain rows with identical values in all columns of the index.  These indexes don't enforce any restraints on…

A key is a column or expression on which you can build an index. Follow these guidelines for choosing keys to index: Consider indexing keys that are used frequently in WHERE clauses. Consider indexing keys that are used frequently to join tables in S…

For MyISAM tables, MySQL can create spatial indexes using syntax similar to that for creating regular indexes, but extended with the SPATIAL keyword. Currently, columns in spatial indexes must be declared NOT NULL. The following examples demonstrate…

什么时候需要重建索引 1. 删除的空间没有重用,导致 索引出现碎片 2. 删除大量的表数据后,空间没有重用,导致 索引"虚高" 3.索引的 clustering_facto 和表不一致 也有人认为当索引树高度超过4的时候需要进行重建,但是如果表数量级较大,自然就不会有较高的树,而且重建不会改变索引树高度,除非是由于大量引起的索引树“虚高”,重建才会改善性能,当然这又回到了索引碎片的问题上了. 关于索引是否需要重建,Oracle有这么一句话: Generally speaking, th…

触发条件:只需要从索引中就可以取出所需要的结果集,此时就会走索引全扫描 Full Index Scan    按照数据的逻辑顺序读取数据块,会发生单块读事件, Fast Full Index Scan   按照数据块的物理存储位置顺序读取数据块,会发生多块读事件,理论上索引快速全扫描会比索引全扫描要快 官档的解释: Full Index Scan In a full index scan, the database reads the entire index in order. A full…

示例执行计划: postgres ; QUERY PLAN ---------------------------------------------------------------------------------- Bitmap Heap Scan width) Recheck Cond: ((id ) ) )) width) width) ) width) ) width) ) ( rows) > what does "Bitmap Heap Scan" phase…

一. 1.什么是B-Tree indexes? The general idea of a B-Tree is that all the values are stored in order, and each leaf page is the same distance from the root. A B-Tree index speeds up data access because the storage engine doesn’t have to scan the whole tab…

oracle index build online与offline测试环境为oracle 11.2.0.4 --sql test SQL> conn test/test )); begin .. loop insert into test.rb_test values(i,'ok'); commit; end loop; end; / SQL> select count(*) from test.rb_test; COUNT(*) ---------- SQL> create index…

Question:  I have a SQL with multiple columns in my where clause.  I know that Oracle can only choose one index, and I know about multi-column composite indexes, but I do not know how to determine the optimal column order for a composite index with m…

官网对skip index scan的解释: Index skip scans improve index scans by nonprefix columns since it is often faster to scan index blocks than scanning table data blocks. In this case a composite index is split logically into smaller subindexes. The number of l…

14.8.9 Clustered and Secondary Indexes 每个InnoDB 表有一个特殊的索引称为 clustered index 用于存储数据. 通常, clustered index 是主键的同义词. 为了使查询得到最好的性能,插入, 和其他数据库操作,你必须了解 InnoDB如何使用 clustered index 来优化最创建的查询和DML操作对于每个表 1. 当你定义一个主键在你的表上, InnoDB 使用它作为clustered index. 定义一个主键对于每个…

14.2.5.2 Clustered and Secondary Indexes : 每个InnoDB 表 有一个特别的索引称为clustered index 行数据存储的地方. 典型的,clustered index是主键的同义词,得到最好的查询,插入性能,和其他数据库操作, 你必须了解InnoDB 使用clustered index 来优化最常见的查询和DML操作在每个表上. 当你定义一个PRIMARY KEY 在你的表上,InnoDB 使用它作为一个 clustered index. 定义…

在ORACLE中,索引访问/查找(Index Lookup)路径有五种方式,分别为INDEX UNIQUE SCAN.INDEX RANGE SCAN.INDEX FULL SCAN.INDEX FAST FULL SCAN .INDEX SKIP SCAN.下面通过一些案例介绍.总结一下这五种索引访问路径.本文是总结这方面的知识点,所以文中一些地方参考.引用了参考资料中的部分内容.详细.具体资料可以参考官方资料Index Scans 索引唯一扫描(INDEX UNIQUE SCAN)   索引…

[Oacle][Partition]Partition操作与 Index, Global Index 的关系: ■ Regarding the local index and the global index, In the case of "alter table add partition" Even without using "update indexes" and "update global indexes"Local index a…

1.Adaptive Hash Indexes 定义 If a table fits almost entirely in main memory, the fastest way to perform queries on it is to use hash indexes. InnoDB has a mechanism that monitors index searches made to the indexes defined for a table. If InnoDB notices…

Clustered and Secondary Indexes secondary index A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB tab…

一. 1.什么是hash index A hash index is built on a hash table and is useful only for exact lookups that use every column in the index. For each row, the storage engine computes a hash code of the indexed columns, which is a small value that will probably…

转载.节选于 https://dev.mysql.com/doc/refman/8.0/en/innodb-indexes.html This section covers topics related to InnoDB indexes. 1.Clustered and Secondary Indexes Every InnoDB table has a special index called the clustered index where the data for the rows i…

前言 JanusGraph是一个图数据库引擎,安装及入门可以参考 JanusGraph 图数据库安装小记.为了提高查询速度,在使用过程中一般要为某些属性创建索引.这篇随笔主要是记录创建索引过程中踩过的坑. 索引介绍 与mysql创建索引不同,JanusGraph的索引有一套生命周期,如下图所示: 我们的目标是从<create>索引开始,通过一系列action,最终使索引进入ENABLED状态. 下面简单说明以下各个状态及操作: States(SchemaStatus) INSTALLED   …

原文地址:https://dev.mysql.com/doc/refman/5.7/en/explain-output.html 9.8.2 EXPLAIN Output Format The EXPLAIN statement provides information about the execution plan for a SELECT statement. EXPLAIN returns a row of information for each table used in the S…

SQL Server 中什么情况会导致其执行计划从索引查找(Index Seek)变成索引扫描(Index Scan)呢? 下面从几个方面结合上下文具体场景做了下测试.总结.归纳. 1:隐式转换会导致执行计划从索引查找(Index Seek)变为索引扫描(Index Scan) Implicit Conversion will cause index scan instead of index seek. While implicit conversions occur in SQL Serve…