Use this tag for questions about improving the performance and/or efficiency of database queries.

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6
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1answer
5k views

I want to know about the IOPS (I/O Per Second) and How it influences the DB CRUD operation

I want to get a dedicated server for my Database (MySQL) which runs InnoDB Engine. There is this option in InnoDB capacity that you can define the IOPS in it. Let's say that I have a SATA 72K-RPM HDD ...
6
votes
1answer
2k views

Does the order of columns has a significant effect on INSERT?

Do we experience a noticeable difference in performance for these two queries? INSERT INTO table (col1, col2, col3, col4, col5) ... and INSERT INTO table (col5, col3, col1, col2, col4) ... Do we ...
6
votes
3answers
2k views

Postgres is performing sequential scan instead of index scan

I have a table with about 10 million rows in it and an index on a date field. When I try and extract the unique values of the indexed field Postgres runs a sequential scan even though the result set ...
6
votes
3answers
351 views

Convert date in date list condition to list of date ranges

I want to look for all the records that occur on specific dates. SELECT * FROM table1 WHERE date(column) in ($date1, $date2, ...); However, as many of you, know this kind of comparison doesn't get ...
6
votes
2answers
265 views

Clustered Table Scan Because of “SELECT *”

I have a Records table with over 100 columns and very many rows, and a nonclustered index on 5 fields based on my access paths: CREATE NONCLUSTERED INDEX [IX_Records_CustomerID] ON [dbo].[Records] ( ...
6
votes
2answers
1k views

Why is my plan guide not being used?

We recently ran into the tipping point issue and some of our report queries that used to complete execution within a couple of seconds are now taking more than 2 min because the query optimizer simply ...
6
votes
1answer
15k views

MySQL: Optimize UNION with “ORDER BY” in inner queries

I just set up a logging system which consists of multiple tables with the same layout. There is one table for each data source. For the log viewer, I want to UNION all the log tables, filter them ...
6
votes
3answers
436 views

Slow query performance due to temporary file?

Here is the query: SELECT "products".* FROM "products" WHERE (status > 100) AND "products"."above_revenue_average" = 't' AND ("products"."category_id" NOT IN (5)) ORDER BY "products"."...
6
votes
2answers
1k views

Filter table before join

I have a stored procedure that populates the temp table #employee_benefits with a list of Ids. This table ends up being roughly 10,000 rows long. The query below then selects from a table called ...
6
votes
1answer
247 views

Is it recommended to clear the query plan cache

Pretty much as the title describes. I have just checked a SQL Server and noticed there is a lot of query plan bloat that can be fixed by proper parametrisation. After making the changes to the code, ...
6
votes
1answer
13k views

Optimizing large database query (25+ million rows, using max() and GROUP BY)

I'm using Postgres 9.3.5 and I have a large table in the database, currently it has over 25 million rows and it tends to get even larger rapidly. I'm trying to select specific rows (all unit_ids with ...
6
votes
1answer
10k views

Comparing two queries in SQL Server 2012

I am comparing two queries in SQL Server 2012. The goal is to use all of the pertinent information available from the query optimizer when choosing the best query. Both queries produce the same ...
6
votes
1answer
382 views

Landed as BI, but databases are a big WTF, what to do?

Maybe a duplicate, but I believe my case is a bit different. From one of the answers I got to this post on SQL Server Central that also comes handy too but is not quite the same scenario: 9 Things to ...
6
votes
1answer
6k views

mySQL query optimisation — multiple joins or select … where not in (select distinct…)?

Background I have a Drupal install accessing a large users database (~200k rows) and my "People finder" functionality needs to access all those rows (in a random order). I don't seem to be able to ...
6
votes
1answer
139 views

6GB Table - 37Mi Rows - 'System.OutOfMemoryException' - SQL 2008 R2

I have a proc, looping through databases( the insert is in a fix database...the select uses the loop ), making inserts (Will hide names for security): insert into Main_Database..Table(16 Fields) ...
6
votes
1answer
7k views

Could too many idle connections affect PostgreSQL 9.2 performance?

Some queries on my database server seem to take a long time to respond, and I believe the CPU usage is high. When running ps aux, I see ~250 "idle" connections (which I believe to be too many). I ...
6
votes
2answers
2k views

Optimisation of complex query - 100% cost index seek?

I'm a c# developer, however I'm quite experienced with SQL. I have had a bit of experience with optimising queries but I've come up across one that has me stumped. The query takes about 3 minutes to ...
6
votes
1answer
73 views

Is it useful to split a table into a frequently used and a archive table?

I have a table with 1.000.000+ entries. Only the newest 100.000 entries are used very frequently. The other 90% are used very seldom. Is it useful to split this table into a frequently used table ...
6
votes
2answers
561 views

Postgres performance difference on insert

I was tried to partition one large table, and come with such Postgres behavior that I could not explain. Maybe you will have any suggestions? I went quite straight forward way (i know that it is not ...
6
votes
1answer
340 views

Performance of query with a range condition and order by

We have the following table (in SQLite on Android) which a tree structure (Nested Set model) of words and their frequencies: lexikon ------- _id integer PRIMARY KEY word text frequency integer ...
6
votes
3answers
802 views

Single query sending and retreiving duplicate data on MS SQL Server?

Very odd thing happening. It would appear that all queries against a particular database in our system are periodically "running slow". Ie "normal speed" for 5 minutes then slow for 5 minutes (roughly)...
6
votes
2answers
115 views

Query Processing & The Buffer Pool

When SQL Server Server 2008r2 begins to process a query, I know it needs to parse the SQL Text and create the plan, plus a few other steps. It is doing all of that in memory OUTSIDE of the buffer ...
6
votes
3answers
4k views

Slow queries on billions-rows-table // index used

Since I'm a young developer and not really skilled in using databases (PostgreSQL 9.3) I ran into some problems with a project, where I really need help with. My project is about collecting data of ...
5
votes
3answers
492 views

Warning about implicit convert

I have table with 2 name columns: CREATE TABLE Test ( TestID int identity primary key clustered , Name_Eng nvarchar(50) , Name_Nat nvarchar(50) ) Now I need a query to get this Name column ...
5
votes
2answers
10k views

Why won't SQL Server optimize the UNIONs?

Consider these queries (SQL Fiddle): Query 1: SELECT * INTO #TMP1 FROM Foo UNION SELECT * FROM Boo UNION SELECT * FROM Koo; Query 2: SELECT * INTO #TMP2 FROM Foo UNION SELECT * FROM Boo UNION ALL ...
5
votes
3answers
2k views

Some book suggestions for performance tuning

I am in the process of improving performance on our database application. While I am not a DBA, I am pretty comfortable with SQL. I am after a book that can help me understand how queries written in ...
5
votes
1answer
2k views

SQL Server, TOP versus ROW_NUMBER

I'm learning about Execution plans and am trying out different queries and comparing their performance and stumbled upon this: SELECT StatisticID FROM ( SELECT StatisticID, ROW_NUMBER() OVER (...
5
votes
2answers
294 views

Cannot tune database any further; what next?

We have a vendor-supplied application. It is under support and we are talking to their developers (we have their largest database size by an order of magnitude), but in the meantime, we have the ...
5
votes
2answers
334 views

Store categories in database or use hard coded integers?

An address can belong to either the mailing or billing categories. When a user adds an address to a table, should the address category be an independent integer value set by a constant somewhere in ...
5
votes
4answers
4k views

MySQL subquery slows down drastically, but they work fine independently

Query 1: select distinct email from mybigtable where account_id=345 takes 0.1s Query 2: Select count(*) as total from mybigtable where account_id=123 and email IN (<include all from above ...
5
votes
3answers
617 views

Speed of MySQL query for the most recent 800 entries in chronological order

I have a MySQL database running on an Ubuntu box that pings 250 clients once a minute (24x7). My column names are: ip_address varchar(16), status varchar(7), timestamp datetime Been running it for ...
5
votes
2answers
10k views

Are there advantages to using temporary tables over derived tables in SQL Server?

I read the derived tables have better performance than temporary tables, but anyway many SQL Server developers prefer the second ones. Why? I must do queries with large data (millions records) and I ...
5
votes
4answers
1k views

optimizing a compare query

Suppose I have two tables, A and B, and I know that size(A) = size(B). I want to confirm that the data in both tables is the same in three given columns, suppose they are X, Y, and Z (there are no ...
5
votes
2answers
4k views

Does a TRIGGER improve the performance?

I understand that TRIGGERs are useful, as they take care of subsequent queries, but do they have an advantage from performance point of view too? Replacing two or three queries as INSERT INTO table1 ...
5
votes
1answer
452 views

Logical reads amount difference with almost identical indexes

I'm trying to figure out why two identical queries are taking slightly different times when executed under two indexes that are almost identical, in SQL Server. The mentioned query is quite simple, ...
5
votes
1answer
87 views

Will these queries return the same results?

I have this simple query: SELECT friend_id FROM default_friend WHERE user_id = 1 And the results are below: +-----------+ | friend_id | +-----------+ | 2250 | | 4901 | | ...
5
votes
1answer
2k views

Scaling PostgreSQL TRIGGER(s)

How Postgres triggers mechanism scales ? We have a large PostgreSQL installation and we are trying to implement an event based system using log tables and TRIGGER(s). Basically we would like to ...
5
votes
1answer
221 views

When does an execution plan not show in sp_WhoIsActive?

I use Adam Machanic's excellent sp_WhoIsActive procedure to see activity and to log to a table to troubleshoot long running queries. Why does the execution plan not show all the time in the results? ...
5
votes
2answers
90 views

Help finding join without predicate

Much like a related question by swasheck, I have a query that historically has suffered from performance problems. I was looking through the query plan on SSMS and noticed a Nested Loops (Inner Join) ...
5
votes
2answers
109 views

looking for missing indexes in the cached execution plans

I have the following query that shows me what is cached in sql server: SELECT cp.objtype AS ObjectType, OBJECT_NAME(st.objectid,st.dbid) AS ObjectName, cp.usecounts AS ExecutionCount, st.TEXT AS ...
5
votes
2answers
1k views

MySQL queries are 10 to 100 times slower after OS reboot

I have a MySQL server running on Ubuntu. If I restart the MySQL, everything is fine. If I however restart the OS, my queries take anywhere from 10 times to 100 times as long. The only "solution" to ...
5
votes
1answer
1k views

Slow SQL Server query using a DB2 linked server in virtual environment

I have a problem with a slow query which uses a linked server. Let me list out my environment first and I'll get to the issue: Virtual Windows 2003 R2 server (fresh deployments on XenServer 6.1 &...
5
votes
2answers
9k views

Help to understand explain plan in Oracle

I am running a query in some big tables, and although it runs fine even tough is a lot of data, I'd like to understand what part of it weighs on the execution. Unfortunately I am not too good with ...
5
votes
2answers
874 views

Why is a secondary index chosen over a clustered index for SELECT COUNT(*) …?

In this query: select count(*) from largetable; a secondary index is chosen: mysql> explain select count(*) from largetable; +----+-------------+------------+-------+---------------+------+-----...
5
votes
1answer
156 views

Indexes: integer vs string performance if the number of nodes in the index is the same

I hope this is the right place to ask! I am developing an application in Ruby on Rails with the PostgreSQL (9.4) database. For my use case, columns in tables will be looked up very frequently, as the ...
5
votes
3answers
411 views

Should I split this large table into three smaller tables?

I'm designing a PostgreSQL database that will contain information about photos uploaded by my users. All users will have at least one main photo and optionally one or more public and/or private ...
5
votes
2answers
246 views

how to increase speed of this mysql query?

How I can increase speed of this search query? This search bring news based on time and title and text. each item has own factor. SELECT * , MATCH (`title`) AGAINST ('tax in work') * 1.65 AS `...
5
votes
1answer
204 views

Query looks optimal but logical reads is huge

I run the following (really simple) query in SQL Server: SELECT MAX(PK_Field1) FROM MainTable kh WHERE kh.PK_Field1 >= '2014-12-01T00:00:00' AND kh.PK_Field2 = 1572 AND kh....
5
votes
2answers
147 views

Sporadic execution time causing timeouts [closed]

We have a table with 100 million records holding 24 hour worth of data of our clients' customers actions (actions which are performed on 3rd party sites and replicated from there to our server) The ...
5
votes
1answer
943 views

How does order of index affect performance in SQL Server?

I am wondering how does order of index affect performance (ASC, DESC) ? If it affects, why ? And which order should I choose ?