根据以上现象推断,PostgreSQL 似乎在 count 的数据量小于数据表长度的某一比例时,才使用 index scan,通过查看官方 wiki 也可以看到相关描述: It is important to realise that the planner is concerned with minimising the total cost of the query. With
akendb=# CREATE FUNCTION count_estimate(query text) RETURNS integer AS akendb-# $func$ akendb$# DECLARE akendb$# rec record; akendb$# rows integer; akendb$# BEGIN akendb$# FOR rec IN EXECUTE 'EXPLAIN ' || query LOOP akendb$# rows := substring(rec."QUERY PLAN" FROM ' rows=([[...
Postgresql支持变长参数传递，参数被自动转换为数据传入函数体中，类似C语言的可变参数：int sum(int num...
it just hasn't been done yet. It is complicated by the fact that you can have multiple count() expressions in the same query which demand sorting/grouping on different columns.
Visual Presentation of PostgreSQL COUNT with HAVING PostgreSQL COUNT with GROUP BY and ORDER BY Sample table: employees The following query will return the designation where at least 5 employees are working with a maximum salary below 12000 and the number of employees for each designation in descend...
InnoDB handles SELECT COUNT(*) and SELECT COUNT(1) operations in the same way. There is no performance difference. postgresql 的官方文档中也有关于 count 的说明: 在把count聚集应用到整个表上时,习惯于使用其他 SQL 数据管理系统的用户可能会对它的性能感到失望。一个如下的查询: ...
reltuples provides an estimate of the row count for each table in PostgreSQL. Examples and Code Explanation: 1. Using pg_stat_user_tables to Retrieve Exact Row Counts Code: -- Select table names and row counts from pg_stat_user_tables ...
那么这个对你绝对有用哦...php// 添加开始时间函数function runStartTime(){ define('RUN_STARTTIME', microtime(true));}// 计算耗时和查询数据库次数并输出函数...$databaseLink = MySql::getInstance(); $queryNum = $databaseLink->getQueryCount(); echo "页面加载耗时...秒查询数据库:".$queryNum...
Often an approximation is good enough and you don't need the exact count. In that case you can use the estimate that PostgreSQL uses for query planning: 1 2 3 SELECTreltuples::bigint FROMpg_catalog.pg_class WHERErelname='mytable'; ...
I can't see a reason why all columns are SELECTed in a subquery just to perform a simple count. I've attached the SQLs generated and what I would expect. So, for any model, a query like this: Model.objects.distinct().count() should produce an SQL like this: SELECT COUNT(*)...