In the context of a database, optimisation refers to the process of the query optimiser selecting an efficient physical execution plan.

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Maximum possible memory usage: 16.2G (874% of installed RAM)

Why the MySQL is saying that can use about 16GB of RAM even if my server has only about 2GB? # ./mysqltuner.pl >> MySQLTuner 1.2.0 - Major Hayden <major@mhtx.net> >> Bug ...
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Searching for pairs of date/time values in a single row

I have a legacy IBM DB2 database table that contains lots of records of phone calls. It has columns for ID, customer number, employee who took the call, etc. The date/time for each call is kept in a ...
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Mysql Index for concat select

For Below Query SELECT MAX(CONCAT(date, ' ', last_entry)) AS LAST_LOG FROM entry_log WHERE TRIM(LEADING 0 FROM card_no)='2948' I've index on date card_no date,last_entry ...
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Optimize a query on two big tables

I have a very important query in my system that is taking too long to execute due to huge amount of data on tables. I'm a junior DBA and i need the best optimization I can get for this. Tables have ...
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Analysing A Query Plan

Why is the following query slow? select count(*) from [dbo].[mt_dispatch_link] , [dbo].[_mt_dispatch] [_mt_dispatch] where (mt_dispatch_link.contract_id_1 = _mt_dispatch.contract_id ...
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Optimising MySQL

As per this post, it looks like I'm having some serious issues with MySQL optimisation. Unfortunately, I have no idea where to start! I'm running a DirectAdmin-based CentOS dedicated server. It's an ...
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How to improve MySQL Server Performance..??

As a MySQL DBA most of the times we are supposed to optimize a poorly performing MySQL Servers. Now my question is where to start from like we will need to find out many things as 1.Find the ...
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Optimisation Newbie: How much of a 'sin' is redundancy?

I've been developing for a while, but never really had to deal with DB/scaling issues before. That's suddenly changed and I've found myself in the deep end. I have 2 SQL tables, as such: VOTES ...
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How can I optimize performance on a MySQL ndbcluster?

So, this is what I have: A simple table classes__to_students: |class |student| --------------- Math Alice Math Bob Math Peter Math Anne Music Bob Music Chis Music ...
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Moving data from table with VARCHAR(50) fields to table with numeric fields increases table size

I want to understand why migrating data in a table with all VARCHAR(50) fields to a table with optimized smaller types caused the new table (containing 61,065,164 rows) to be 4.46 GB, which is larger ...
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730 views

Has postgresql a fast add column optimization?

When adding not-null columns with a default value - does Postgresql optimizes this operation? In case the table has n rows, a not-optimized alter-table-add-column would yield n writes of the default ...
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cross database way to force join strategies

[how to enforce the query to use certain join algorithm] Knowing that there are several joining algorithms [Nested Loop; Sort Merge; Hash Join], when implementing a query , is there any technique to ...
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430 views

How un-clustered is a CLUSTER USING table in PostgreSQL

I have some tables which benefit greatly from CLUSTER ON/CLUSTER USING in Postgres SQL: # CLUSTER table USING index_name; # ANALYZE VERBOSE table; # CLUSTER VERBOSE; A maintenance task periodically ...
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Correct use of VOLATILE COST (and ROWS) indications in Postgresql stored procedure

While looking at several examples of pl/python and pl/pgsql, I have seen many - but not all - using volatile cost. ie: CREATE OR REPLACE FUNCTION my_function() RETURNS setof record AS $BODY$ -- ...
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MySQL optimization - year column grouping - using temporary table, filesort

I have a transactions table which is having 600,000 records, I need to list the count for the dashboard on financial year basis. The table used is MyISAM. I tried adding index for the transaction date ...
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How to optimize this dynamic stored procedure

I have a stored procedure which is retrieving data using 20 tables. Sample of the procedure: CREATE PROCEDURE GetEnquiries ( @EnquiryDate DATETIME = NULL ) AS ...
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What are the key points to avoid table locks?

MySQL is using at my company and most of the database administration works on trying to solve the table locks. What are the key points to avoid table locks(I mean at database side and even at software ...
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999 views

How to speed up select distinct?

I have a simple select distinct on some time series data: SELECT DISTINCT user_id FROM events WHERE project_id = 6 AND time > '2015-01-11 8:00:00' AND time < '2015-02-10 8:00:00'; And it ...
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Find out beforehand how many records a query has

I develop reports in ExtJS resulting of queries in SQL Server 2012. I use Ext.Grid with paging, therefore I can use offset to limit the amount of records a query will return. But for this ExtJS ...
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This query is choking the server. How can I improve it?

Following query is hogging a lot of resources, as much as that the queries are getting locked down & CPU usage goes through the roof! The MySQL Processes list shows that as the number of requests ...
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What are the best Database Analytics Companies for analyzing several 1000s or Mio. of products,sum them together and analyze them for statistics?

We will get several thousands products in our database. The problem is that one product can have several different names e.g. "iPad 16GByte UMTS schwarz" or "iPad 16GB 3G black" and so on. But they ...
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Indices for a join query with datetime range

Given the following query: SELECT DISTINCT COUNT(DISTINCT "patients"."id") FROM "patients" INNER JOIN "transactions" ON "transactions"."patient_data_id" = "patients"."patient_data_id" WHERE ...
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765 views

DB2 CASE statement faster with parentheses around entire WHEN

Trying to optimize slow queries, someone on my team come across this. Having this in our stored proc: CASE WHEN MYCOL BETWEEN @STARTNUM AND @ENDNUM AND RECTYPE = 'RO' THEN 1 ELSE 0 ...
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Simulate more RAM with SQL Server

Background: I'm in the process of optimizing a Data Warehouse style query with big dimension tables. The optimizer does not like to pick the typical star schema join plan at the moment. When I force ...
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Do MySQL optimizations hold for MariaDB also?

I am a developer who will be starting a new project soon in which database performance will be crucial. The current candidate for the backend is MariaDB. I have worked with MySQL and MariaDB before in ...
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271 views

PL/pgSQL functions and optimizations fences?

I am newbie on DB stuff. And my DB knowledge is very old (around MSSQL2000). All what I remember is, the stored procedures are faster than ordinary queries because it avoid query compilation and ...
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What's the optimal server configuration for fastest inserting in a MySql database in this scenario?

I have a MyISAM table partitioned by range with 90 partitions representing the last 90 days. I expect this table to receive somewhere between 500 million to 1 billion records before i start rotating ...
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233 views

Query optimization with extreme statistic

I have a table: mysql> describe table_1; +---------------+------------------+------+-----+---------------------+----------------+ | Field | Type | Null | Key | Default ...
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How to optimize a log process in MySQL?

In my project, I have about 100.000 users and can't control their behavior. Now, what I would like to do is log their activity in a certain task. Every activity, is one record which includes columns ...
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How to setup mysql on tmpfs?

How to setup mysql on tmpfs ? I am reading a tutorial in french, where the author do it this way : 1.) Create a tmpfs directory : mkdir /var/tmpfs mount -t tmpfs -o size=1G tmpfs /var/tmpfs chown ...
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How does the MySQL Query Optimizer react to a SELECT COUNT sentence?

Let's say we have a table called "customers" with these columns ("id", "name", "country", "created_at). There's an index by ("country", "created_at"). If SELECT COUNT(*) FROM customers WHERE ID ...
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How to restructure this slow query containing subquery?

This query takes about 2-6 seconds to run depending which tag id(s) is/are selected: SELECT t2.id, t2.name, count(*) as kpl FROM (SELECT pt1.post_id FROM posts_tags pt1 WHERE pt1.tag_id ...
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Question about cost optimization formula for SELECT

I am a student in computer science and I have a question, which refers to cost optimization. I have to read the book (Fundamentals of Database Systems from Elmasry and Navathe) and try to present the ...
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IN Clause - listing values VS query

My app runs a lot of queries like this SELECT ... FROM Table WHERE CategoryID IN (SELECT CategoryID FROM OtherTable WHERE ..) To save performance I decided to cache the second subquery in the ...
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Optimizer gets the wrong number of estimated rows even with updated stats [closed]

I have a procedure running in SQL Server 2008 that contains a query similar to the one below; SELECT /* various columns from table 1 and table 2 */ FROM dbo.tbl_1 AS a INNER JOIN dbo.tbl_2 AS b ON ...
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Improve performance of query with multiple group by column and count function in select

Brief Background of issue We have a aggregate table to store activities of competitors and this table have currently 3 Million activities and we need to retrieve data from this table group by ...
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Postgres reduce by key if key is not null

I need to select N rows with seq > X which reduce_key is null and for each row group with the same reduce_key select the one with the biggest seq. Result should be strictly sequential (but with ...
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Should I normalise this database?

I have two database designs thought of for storing messages(instant messages) between users. It will be a group chat system. I have two table designs in mind. One is this: CREATE TABLE ...
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Mongodb high lock percentage / slow queries

I'm working with an application using MongoDB that is getting a fairly large number of incoming requests and I've noticed that the responses are getting slower as the number of requests get higher. ...
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Why does the CBO eliminate `distinct` and not `group by` in this query?

The CBO chooses to eliminate the distinct in the 'slow' query - presumably it can tell that the operation isn't needed because of the outer group by. My questions are: 1) Why does it choose to do so ...
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585 views

EXPLAIN ANALYZE shows no details for queries inside a plpgsql function

I am using a PL/pgSQL function in PostgreSQL 9.3 with several complex queries inside: create function f1() returns integer as $$ declare event tablename%ROWTYPE; .... .... begin FOR event IN ...
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When is mariadb creating tmp tables?

I have found, that copying to tmp table is taking most of the time of a slow query, that I wanted to optimize. I watched the output of 'profiling' in a terminal. This happens on a pics table with 6000 ...
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How to optimize this slow SQL query

I have a slow SQL query on my Drupal website and I don't know how to optimize it. Here is the query that Views produces: http://pastebin.com/StAEiXbY SQL fiddle: http://sqlfiddle.com/#!2/20081/1/0 ...
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Designating databases to certain CPUs

SQL Server 2012. AlwaysOn Cluster with two nodes, each has 4 cores We are consolidating SQL server instances and moving bunch of databases from various locations to a single cluster which has two ...
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216 views

Mysql Fragmentation. How bad is too bad?

I ran a table status for my companies production site and its showing close to around '49085939712' for just about every table in my database and we have around 400 tables. I am not a DB ...
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PostgreSQL + query planner + amount of entities

I'm working on a college assignment about how the query planner uses statistics and makes the most optimal query plan. I have read the 57.1. Row Estimation Examples article and I know how PostgreSQL ...
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Timestamp Difference - Syntax Error - Condtitional Selects

Here is what I have right now SELECT name, admin, reason, unban, FROM_UNIXTIME(date) AS datetime, HOUR(difference) AS HRemain, MINUTE(difference) AS MRemain FROM ...
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Having both a integer Primary Key and a GUID/UUID collumn on same table-is this bad

I have my MySQL database having tables where on each table, we have a ID (int,auto increment) Clustered Primary key and a UUID(varchar36) collumn thats just a GUID. The ID is there just for indexing ...
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4m Rows in MySQL, ~5 Minutes to Run Query

I have 2 tables, a users table and a log table which literally logs each time a user signs in. What I'm trying to do is grab a list of all users with a certain rank who have not logged in for over a ...
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MySQL query WHERE IN + ORDER BY, which EXPLAIN is better and how to avoid filesort?

This is my table definition, without any special index for now: CREATE TABLE `filter` ( `field_name` varchar(32) COLLATE utf8_unicode_ci NOT NULL, `category_id` varchar(32) COLLATE ...