A class of issues where a database executes a query inefficiently due to the optimiser selecting an innapropriate query plan.

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59 views

How can I optimize this query?

I'm trying to select the time field from five tables and return the oldest time. The userid field in every table is indexed. Here's the query: ( SELECT `time` FROM `eventlog` WHERE userid = ...
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1answer
301 views

MySQL with slow queries

I have a website running on Wordpress Multisite + Woocommerce with over 5700 users on a Nginx + Apache server. I recently experienced some down time but didn’t get any errors, just a blank page. I ...
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489 views

Different thread states in query execution

My MySQL 5.1 based Intranet system is facing very bad performance during high load. When observed through MonYog, I found below thread states: We can simply understand the different thread state by ...
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1answer
64 views

Poor query performance in MySQL

I have a 10 million record table in a MySQL 5.5 InnoDB. It's running in a 16GB RAM server with good CPU and fast HD. When I run the following query SELECT DISTINCT knowledge_id, ...
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1answer
348 views

Slow running query with inner SELECT in where clause

I have a query that looks like the following: USE DATABASE3; GO SELECT t1.COL1 AS COLUMN1, t2.COL2 AS COLUMN2, t1.COL3, t1.COL4 AS COLUMN4, t1.COL5 AS COLUMN5, ...
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2k views

How to use LOW_PRIORITY for heavy queries?

I have scheduled heavy UPDATE queries at administrator level, and since they are heavy UPDATEs with multiple JOINs, consume a huge amount of resources. I do not want them to stop end users from ...
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1answer
81 views

Query tuning - performance

Could you please help me tune this query. It takes over half an hour to run! I want to identify repeat bookings (can only be identified using an annoying varchar field - email) UPDATE #Repeats --a ...
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1answer
1k views

Using OFFSET in a large table by a simple sub-query

The slow performance for using OFFSET in large table has been widely discussed in various blogs, and the most efficient way is to use an INNER JOIN as SELECT * FROM table INNER JOIN ( SELECT id FROM ...
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1answer
380 views

Identical query, tables, but different EXPLAIN and performance

I have two tables levels and users_favorites +--------------------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | ...
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1answer
970 views

SQL Server performance for tables having large number of columns [closed]

I have a requirement where I need to create a table with about 600 columns. Out of 600 columns about 200 columns will be XML columns. Other column will be of other datatypes such as int, varchar, ...
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1answer
4k views

Different execution plan for the same query if I change a value in the predicate

I am getting two different execution plans for the same query only by changing the value in the predicate, the first one uses nested loops and the other one hash join which takes a lot longer. ...
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2answers
242 views

MySQL (InnoDB) UPDATE SET performance with value in WHERE AND clause

If an UPDATE query is performed where the values being SET are also part of the WHERE clause, in the form of an additional AND statement on the column being set's value, is there a significant ...
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2answers
6k views

INSERT INTO with SELECT subquery db2 9.5

I have the following query that takes 4 seconds to execute: SELECT TX.* FROM TABLE_X TX INNER JOIN TABLE_Y TY ON TY.VALUE_ID = VALUE_ID_IN AND TY.PROD_ID = TX.PROD_ID WHERE (TX.PLACE_ID1 ...
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1answer
522 views

“copy to tmp table” for 24 hours, then timeout; when ALTERing to add PK & AI

I downloaded the Wiktionary XML dump, and ran mwdumper.jar to insert the data into MySQL. Following the advice in the manual, I commented out the PRIMARY KEY AUTO_INCREMENT part of the CREATE TABLE ...
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3answers
2k views

Oracle query is slower the second time

I have a general question about something I'm seeing in performance tuning in Oracle. I have a query that I'm testing. It's too big and complex for me to provide a simple example, sorry! (I tried ...
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1answer
61 views

What ways are there to limit the processor / ram requirements of a specific query?

Every once in a while, I need to run some very expensive query on a production server. Think spaghetti joins. This is likely to affect the server operation, and I don't want that. What ways are there ...
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1answer
3k views

Get top most parent by nth child id?

Now there is a question we commonly use this technique to maintain the parent child relation i.e we store all the entities in one tables with a parent_id column and all top most parents have 0 in the ...
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2answers
889 views

Obvious inefficiencies in this Netezza query?

Why might this query be taking nearly 2 hours to run when similar queries complete within a minute? CREATE TABLE T3 AS SELECT A.*, COALESCE(C.FIELD1,0) AS FIELD1A, ...
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1answer
31 views

Simple query not using indexes

My query is not using indexes. It is a simple query. Can someone explain why that's the case? The query: select TIME_FORMAT(start_time,'%H:%i') start_time, TIME_FORMAT(end_time,'%H:%i') ...
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1answer
216 views

Avoiding performance hit from GROUP BY during FULLTEXT search?

Is there any clever way to avoid the performance hit from using group by during fulltext search? SELECT p.topic_id, min(p.post_id) FROM forum_posts AS p WHERE MATCH (p.post_text) AGAINST ('baby ...
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1answer
308 views

Simplify and optimize a complex query

Our site has a chronological event feed that uses MySQL. We have noticed that some of the queries tend to run for quite a bit longer than others. For example, some queries for our heaviest users take ...
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1answer
2k views

Query performance of a NULL vs an empty ('') varchar in SQL Server 2012

I have 3 indexed fields in a query: int, int, and varchar(250). The query performs well when all 3 conditions are specified with real values. The int columns always have values, but there are plenty ...
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1answer
370 views

Query tuning help needed (Hash Match and Table Scans)

I need help optimizing the following query (returning ~8k rows): SELECT A.sys_id, 'AppSvrRels' = CAST(SUBSTRING((SELECT (', ' + T.name) FROM ( ...
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1answer
2k views

Slow complex query with group/order

I have a problem with a very important query in my application. It is very slow when adding group and order to it. So the full query is: SELECT m.id, m.title, m.title_text, ...
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1answer
222 views

What would be a optimal index for this query in mysql

I'm trying to optimize this query. SELECT * FROM sales_reps WHERE account_id = 1 AND (state = 'enabled') AND (clients >= 5 OR revenue >= 10000) ORDER BY name, platform, client_group, ...
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2answers
596 views

Simple queries taking ages to run after changing from MyISAM to InnoDB

I have a quite large (300GB) table on MyISAM, which I've "converted" to InnoDB, and performance for a specific query has dropped through the floor. When I say "converted", I can't afford to take the ...
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1answer
173 views

Get and count all blog posts and all users that fits a LIKE condition (two sepparated queries)

I've this tables: -- -- Table structure for table `default_cmvblog` -- CREATE TABLE IF NOT EXISTS `default_cmvblog` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(100) COLLATE ...
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1answer
122 views

Several values from different tables

I'm totally lost here and not know how to get this to work and hope any can help me here. I have several tables: default_users, default_profiles, default_status, default_comment and latest ...
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1answer
3k views

PostgreSQL 9.1 - Query on VIEWS taking a lot of time

Using PostgreSQL 9.1, we are having problems while executing queries on a VIEW of PostgreSQL. Following is the situation: We have a partitioned Table "buz_scdr" over which we have built a VIEW ...
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1answer
147 views

Reviewing all new MySql queries on staging environment

The problem I am trying to solve: It's well known that not all developers know what is EXPLAIN in MySql, and not every knows how to use it. Large part of them consider MySql as a black box, no matter ...
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1answer
43 views

Get wall_post and profile data for several users

I'm trying to build a query for fetch wall_posts and profile fields for several user_id. First see the images at: http://www.dropmocks.com/mBjQ7P. The table with id_user_rq, id_user_ap is the table ...
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215 views

MySql - How can I improve this query?

I have the following table: http://pastebin.com/XH6FvhZ8 and the following query: SELECT * FROM (SELECT * FROM `job_logs` ORDER BY id DESC) AS job_logs_tmp WHERE (job_logs_tmp.user_id = ...
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1answer
481 views

Oracle Benchmarking (SQL_NO_CACHE …)

I want to benchmark some queries on an oracle System from a JDBC application. However, the database seems to cache at some point in time. However, this is not intended in this case. Is there ...
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2answers
130 views

Improving MySQL efficiency

I have a table now containing over 43 million records. To execute SELECT, I usually select records with the same field, say A. Will it be more efficient to divide the table into several tables by ...
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0answers
14 views

Select SQL_Calc_Found_Rows query extremely slow

I'm Wordpress 4.2.2 on an Ubuntu 14.04 LTS server, with PHP 5.9x and MariaDB 10.0.17. When clicking on a post on my site I see many slow queries that all resemble this and it ends up taking as 8 or 9 ...
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0answers
24 views

Delete using join or using sub-query?

I need to delete rows from a table based on what is present in a temporary table. For me, both of these statements work : DELETE from main_table where `id` in (select `deletable_id` from ...
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0answers
49 views

Query execution taking long time

SELECT sqltext.TEXT, req.session_id, req.status, req.command, req.cpu_time, req.total_elapsed_time FROM sys.dm_exec_requests req CROSS APPLY sys.dm_exec_sql_text(sql_handle) AS sqltext select ...
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0answers
56 views

INSERT INTO with TABLOCK taking longer than without TABLOCK [closed]

I'm doing something like below to ensure the insert is minimally logged: insert into AnotherDatabase.dbo.HeapTable with (Tablock) ( <column fields ) select <column fields> from ...
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57 views

How to speed up for huge tables in SQL select query?

I have the following huge tables. Table_1 with 500000 (0.5m) rows Table_8 with 20000000 (20m) rows Table_13 with 4000000 (4m) rows Table_6 with 500000 (0.5m) rows Table_15 with 200000 (0.2m) rows ...
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0answers
47 views

Postgres not using the index even when rows returned is 5% of the table

I have a postgres table with the following structure: +---------+-------------+-------------+----------+---------+---------+ | id | timestamp | numvalues | text1 | text2 | text3 | ...
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1answer
51 views

Why MySQL SELECT performance depends on table size? [closed]

I've got two identical tables with the following structure: CREATE TABLE words ( id bigint(20) NOT NULL, `text` int(11) NOT NULL, w1 int(11) NOT NULL, w2 int(11) NOT NULL, w3 int(11) NOT ...
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0answers
52 views

Most effective way to manage a growing list of time series data in an MS Access database

I am building a database to store time series data along with some other summary statistics. Since this is a local application I do not have access to more robust database solutions and am confined to ...
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39 views

PostgreSQL DB freezes with large inserts

I am developing a web application with an SQLAlchemy ORM linked with PostgreSQL database backend. I have this usecase where a client uploads a file of about 20 MB containing large data. What I am ...
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63 views

Query Performance - Why is a query slower when there are two criteria on the same column?

I have these two queries against a view, where I've added WHERE clauses against a date column (Indexed, Unique, Non-Clustered, Ascending): SELECT ColA FROM MyView a WHERE a.[Date] >= ...
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1answer
99 views

MongoDB document size for collection - impact on RAM and query performance

I have been advised on stackoverflow to ask this question on dba.stackexchange instead - We are using MongoDB version 3.0 with WiredTiger storage. As newbies to MongoDB, we may have designed our ...
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57 views

Is there a way to improve this query further?

I've got a query that over time I've been improving. Originally it simply selected * from the records table, and the data was formatted on the webapp side. But that was slow and I wanted to make ...
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36 views

Poor performance filtering mysql spatial join

First of all, sorry for the crossposting. I ask this question in stackoverflow and they sent me here I have a mysql table with spatial references (GEOGRAPHIC_FILTER), like cities, countries, natural ...
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38 views

MDX query is SLOWER with warm cache

I have a MDX query that executes faster on cold cache than warm cache. If I ran the same MDX query after restarting SSAS it takes 14 seconds. All subsequent runs of the same statement then take 50 ...
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0answers
103 views

Simple avg query on large table much slower in PostgreSQL than SQL Server

I have a database with three tables items, parameters and measurements in both servers and want to query the measuerment table. But the query is much slower in PostgeSQL (9.4) vs SQL Server (2012). ...
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26 views

Sampling and matching samples with spatial conditions

The Problem I have two tables with start points of a route and possible end points of a route which need to be matched against each other. I also have a given distribution for the distance between ...