COUNT DISTINCT 的性能是PostgreSQL中长期存在的问题, 在版本14中尚未解决. 在数据量大的时候, 这个查询会很慢, 千万级别的表可能需要10秒左右才能返回结果 SELECT COUNT(DISTINCT field_1) FROM table_1 1. 2. 3. 4. count(distinct …) always sorts, rather than using a hash, to do its work. I do...
MySQL 是不支持 select ... into ，但是可以使用 insert into ... select 当然也可以使用 create ...
chunk_data - 实际存储的数据 插入数据 INSERT 0 1postgres=# insert into test_varlena select 1,'aaaaaaaaaa','ABCDEFGHIJKLMNOPQRSTUVWXYZ',repeat('abcdefghijklmnopqrstuvwxyz',5); INSERT 0 1 postgres=# insert into test_varlena select 2,'bbbbbbbbbb','ABCDEFGHIJKLMNOPQRSTUVWXYZ',repeat('abcdefghijkl...
updateas a_horse_with_no_name suggests，union是选择DISTINCT值的方法，因为这里首选UNION ALL-以防万...
创建btree索引时,btbuild的时候,_bt_spools_heapscan中会创建spool,其中存放了tuplesortstate,tuplesortstate中存放的是索引数据。 typedef struct BTSpool { Tuplesortstate *sortstate; /* state data for tuplesort.c */ Relation heap; Relation index; bool isunique; bool nulls_not_distinct; } BTSpool;...
说明 是否必须使用 SELECT 要返回的列或者表达式 是 FROM
First of all, we're going to talk about the new unique constraint NULLS NOT DISTINCT option, and then we'll talk about the SELECT DISTINCT performance improvements. Share this episode: Click here to share this episode on twitter, or sign up for our newsletter and check out the newsletter ...
SELECT COALESCE(NULL,'123') != '13'; 10、判断否个字段中是否包含否字符串,类似CHARINDEX(@Param,ColumnName) SELECTdistinctFloorNo,HouseIdFROMpublic.v_SchoolRoomFloorWHEREPOSITION('13000205F100001'INUInstallId) >0ANDHouseId=1ANDStatus=1;SELECTdistinctFloorNo,HouseIdFROMpublic.v_SchoolRoomFloorWHERESTR...
markers found in the statement (for a bind descriptor). IfFcontains a negative value, you can compute the absolute value ofFto discover how many values or parameter markers are required. For example, ifFcontains-24after describing aSELECTlist, you know that the query returns 24 columns. ...
Version 3 of the “Activity Book for Postgres” is hot off the press. And you can get your own copy if you come say hello at the Microsoft booth at the PGConf NYC and PGConf EU conferences in 2023. The...