explain

PostgreSQL的执行计划分析

由 故事扮演 提交于 2019-12-17 12:03:05
【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 近期有人提出想查看Postgresql的执行计划，下面分析下PG执行计划中的cost等相关值是怎么计算出来的： PG的版本是9.1.2 1.终端工具PGADMIN,对执行的语句按F7即可，然后看数据输出和解释 2.命令行分析：explain select * from table_name; 一般我们会比较关注消耗值cost和扫描的方式，如走索引或者full scan全表扫描.当COST值消耗比较大时需要注意是否有优化的可能。 与执行计划相关的几个参数，参看下面的示例： kenyon=# select count(1) from dba.website ;　　　　　　　 --普通堆栈表，无任何索引约束 count ------- 20 (1 row) kenyon=# explain select * from dba.website ; QUERY PLAN -------------------------------------------------------- Seq Scan on website (cost=0.00..1.20 rows=20 width=4) (1 row) --relpages磁盘页，reltuples是行数(与实际不一定相符，一般略小) kenyon=# select

Explain this Kotlin function structure

由 夙愿已清 提交于 2019-12-12 16:44:54
问题 I am working with this Kotlin function. I know that we have a function called mPasswordView!!.setOnEditorActionListener , that take parameter TextView.OnEditorActionListener , but what's that after it? we have curly brackets inside parameter? mPasswordView!!.setOnEditorActionListener(TextView.OnEditorActionListener { textView, id, keyEvent -> if (id == R.id.login || id == EditorInfo.IME_NULL) { attemptLogin() return@OnEditorActionListener true } false }) 回答1: The feature used in your example

How to interpret the output of MySQL EXPLAIN?

由 假如想象 提交于 2019-12-12 08:27:12
问题 I want to select the content of the column text from entrytable . EXPLAIN SELECT text FROM entrytable WHERE user = 'username' && `status` = '1' && ( `status_spam_user` = 'no_spam' || ( `status_spam_user` = 'neutral' && `status_spam_system` = 'neutral' ) ) ORDER BY datum DESC LIMIT 6430 , 10 The table has three indices: index_user (user) index_datum (datum) index_status_mit_spam (status, status_spam_user, status_spam_system) The EXPLAIN result is: id select_type table type possible_keys key

db2 explain result about lock size

由 被刻印的时光 ゝ 提交于 2019-12-12 01:53:22
问题 What is Lock List Size , Maximum Lock List Size and Locks Available in the result of explain using IBM data studio. I read some sources from IBM but can not understand the complete concept and different between those. Please tell any resources about these attributes (also others in the attribute pane) Thanks 回答1: Thanks to Lennart, Lock list size is Lock list memory in use (Bytes) (see snapshot). Maximum lock list size is MAXLOCKS, Percent. of lock lists per application (see db cfg) and Locks

Performace Lag in Mysql query

由 十年热恋 提交于 2019-12-11 14:11:33
问题 This mysql query takes 5 minutes 20 seconds to execute SELECT PROVIDER, COUNT(DISTINCT(NAME)) FROM Test WHERE NAME NOT IN (SELECT NAME FROM Test WHERE OPERATION = 'SIGN_IN' and Test.CREATED_TIME BETWEEN UNIX_TIMESTAMP(CURRENT_DATE() - INTERVAL 1 DAY) * 1000 AND UNIX_TIMESTAMP(CURRENT_DATE()) * 1000) AND Test.CREATED_TIME BETWEEN UNIX_TIMESTAMP(CURRENT_DATE() - INTERVAL 1 DAY) * 1000 AND UNIX_TIMESTAMP(CURRENT_DATE()) * 1000 AND OPERATION='VALIDATE' GROUP BY PROVIDER; The explain give the

Why does mysql show 2 different results on 2 different servers for the same query

由 送分小仙女□ 提交于 2019-12-11 10:57:32
问题 I have the same MySQL SQL statement running on 2 different databases (my local machine and my production machine). The one on my local machine runs faster while the one on production is slow. Here are the EXPLAIN results on each. Local Machine Production Machine I'd rather not post the exact query if I don't have to. The only difference I can tell between the 2 is that my local machine is running version 5.6 while the production server is running 5.5. Also, the data on my server is 3 days old

Slow Postgres 9.3 Queries, again

由 ≡放荡痞女 提交于 2019-12-11 08:15:35
问题 This is a follow-up to the question at Slow Postgres 9.3 queries. The new indexes definitely help. But what we're seeing is sometimes queries are much slower in practice than when we run EXPLAIN ANALYZE. An example is the following, run on the production database: explain analyze SELECT * FROM messages WHERE groupid=957 ORDER BY id DESC LIMIT 20 OFFSET 31980; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------------

Store XML explain plan in PostgreSQL database

由 北慕城南 提交于 2019-12-11 02:12:24
问题 How can I store XML explain plan (or any other format) in PostgreSQL database? Test data: explain (verbose, format xml) select 1 Table to store results: create table tmp.explain_plan_data (expp xml); My naive test failed: insert into tmp.explain_plan_data values (explain (verbose, format xml) select 1); It seems that explain cannot be used in any other select statement, the following statement does not work either: select * from (explain (verbose, format xml) select 1) a We are using

MySQL: why 'explain' command yields different results on same SQL statement?

由 我怕爱的太早我们不能终老 提交于 2019-12-11 00:07:43
问题 I migrated a MySQL database from one environment to another, and discovered that after migrating, a particular query runs extremely slower than it was. I was investigating the statement with 'explain' keyword (below) and found that the 'explain' command gives different output, on different servers. explain select distinct j.job,f.path,p.path from fixes f join jobs j on f.job=j.id join paths p on p.id =f.path where p.path like '//depot1/Dev\-trunk/%' ; For the original one, it gives: 1 SIMPLE

PostgreSQL: exists vs left join

由 牧云@^-^@ 提交于 2019-12-10 15:21:24
问题 I heard many times that postgres handles exists queries even faster then left join . http://archives.postgresql.org/pgsql-performance/2002-12/msg00185.php That's definitely true for one table aggregation. But in our case their is more then one and the same query build with exists that make postgres to hang forever: explain SELECT count(DISTINCT "groups".id) AS count_all FROM "groups" WHERE (exists( select * from products p where groups.id = p.group_id AND exists( select * from products
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