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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Is it possible to decouple row size estimation from maximum column width?

In other words, is it possible to declare a column VARCHAR(n) and have the estimated size be something other than n/2? An example use-case might be storing URLs - you expect most to be relative and ...
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1answer
745 views

INSERT take too much time

I have in my MySQL DB a few tables Most of the time, I'm only doing INSERT in my db. I have up to 4 connections to the DB, each one inserting into a different table. There cannot be two connections ...
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236 views

Alternatives for subquery

There are three tables: product_hits, products, and product_subcategory_map. I want to count the number of product_hits and for each product. There could be more than one subcategory in the ...
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1answer
2k views

InnoDB: very bad performance on INSERT/UPDATE

Lately I have really big problems with INSERTs and UPDATEs. On my website 80% of all queries are SELECTs but in query slow log there are only INSERTs and UPDATEs. My dedicated server has 32GB RAM, ...
0
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1answer
296 views

Improve Insert performance for 200 Million rows

How can the following insert statement be improved - which varies from 20mins on certain days to 70-140mins on most other days. SELECT DISTINCT od.SourceID, od.OrderID, od.OrderLineNo, ...
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65 views

Optimize query to minimize tmp copying time

I have this mega query : SELECT s.eleve_id, crs.id_cours as 'cours_id', mc.code as cours_code, s.employe_id, s.id_seance, '0' as theorique, s.statut, s.type_vehicule_id, s.type_conduite_id, ...
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1answer
452 views

Optimize table and query, aggregate by date or date-hour

I have a table storing sensor data (100M rows currently) from stations from different sites. In my case, sites may have many stations. Create table statement is as follows: CREATE TABLE sensor_data ( ...
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2answers
4k views

Optimize a MySQL Database using MySQL Workbench

I actually have a few issues. First, I found this information about optimizing: MySQL OPTIMIZE all tables? The problem is, I don't seem to have a mysqlcheck program anywhere. I'm using MySQL ...
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1answer
43 views

General purpose view for last value in a group

I am trying to create a view to show the last value in the group. Looking for a more general purpose query. CREATE TABLE a ( id INTEGER, seq INTEGER, val VARCHAR2(16), PRIMARY ...
3
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2answers
721 views

What are the options of reducing tempdb latency?

Took the job of monitoring and tununing a SQL Servers 2012 Enterprise Ed. SP1 (64 bit) on Windows Server 2008 R2 Standard Ed. All of them are running inside virtual machines. One SQL Server per a ...
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1answer
798 views

PostgreSQL FTS and Trigram-similarity Query Optimization

I have recently started working on PostgreSQL and I have around 12M rows to handle in which I want to apply Full Text Search. I don't have any prior experience in handling such databases. I have tried ...
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1answer
237 views

Need to optimize simple SELECT query for InnoDB table with 2million rows

I'd like to make this query the fastest possible, since it's called really often. SELECT login_events.id FROM login_events WHERE ( DATE(created_at) >= DATE(CURRENT_DATE) AND person_id = 1 ) ...
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1answer
6k views

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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1answer
87 views

Complex “activity” based query optimization

I'm working on a query in the same style as Facebook's feed on a table that currently has 500,000 rows. For now I am just looking to get the 20 newest items (new records created by a user's ...
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4answers
167 views

Optimize MySQL Query using LEFT JOIN

I have a query with execution plan as below and is taking more than 20 seconds to execute how can i optimize it EXPLAIN SELECT c.coupon_id as c_id, `c` . *, `st`.`name` AS `store` FROM `coupon` AS ...
3
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1answer
109 views

Simple SELECT takes forever, but works as a subquery. Why?

I'm running a MySQL 5.1 server and I have a MyISAM table called my_table with nearly 5 million records. Its structure is the following: ...
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90 views

Query optimization, SQL Server cache query, SQL Server execution query

I have 2 queries, the results of their execution is the same, but I don't know which of these is the best in terms of performance and why. Query #1: SELECT CASE WHEN ( EXISTS (SELECT 1 AS [C1] ...
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1answer
418 views

Why are the plans different if the queries are logically alike?

I wrote two functions to answer Day 3's first homework question from Seven Databases in Seven Weeks. Create a stored procedure where you can input a movie title or actor's name you like, and it ...
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1answer
91 views

MySQL geospatial query optimisation suggestions

I am new to DB Administration. Trying to sort out this query which is the heart of our application. I am searching google on similar questions, but would like an experts opinion on my particular ...
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1answer
106 views

On which fields to index? Fields queried (WHERE) or fields returned (SELECT)

I have a large MySQL database table (~1 million rows and growing) on an AWS RDS Medium instance: mysql> describe clients; ...
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0answers
207 views

is it good to add index on column used in group by clause?

I have a query like- select x, sum(y) from xyz where z>=100 group by x order by sum(y) desc; will it be faster if I am going to add index on x?
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1answer
177 views

Optimizing query on a partitioned table

Tried to find an answer for this, but couldn't. So bear with me and apologies if this has been answered elsewhere. It may be just me not being able to find it. Here's the setup: SQL 2008 R2 EE. Table ...
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2answers
68 views

What does enabling containment on server incur?

Having read in a book of Patrick LeBlank "Microsoft® SQL Server® 2012 Step by Step": "Benefits and limitations of contained databases As with most features, there are benefits and ...
0
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1answer
38 views

Oracle sql plan for a select with procedure in terms section

Is there any difference in optimizing a plan to execute any sql request in case of using a procedure from a package which checks to display row or not and in case of constructing a plain sql query ...
4
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1answer
222 views

SQL query filled Temdb complete Harddisk [closed]

I have a query which was earlier fine but now it is running me out of hard disk space with message that tempdb is full. My Harddisk has 90 gb of space as soon as i run query temdb runs out of space in ...
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1answer
94 views

DROP TABLE on a huge InnoDB table

I have an innodb table with over 140 million rows taking 26GB. I would like to drop this table however this is taking way too long, probably days. How can I speed up this query? processor: Intel® ...
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795 views

Optimizing a CTE hierarchy

Update below I have a table of accounts with a typical acct/parent account architecture to represent a hierarchy of accounts (SQL Server 2012). I created a VIEW using a CTE to hash out the hierarchy, ...
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2answers
257 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
231 views

if database is not shrinked does it affect backup size?

Ok my question is simple. Lets say that when i click database and hover over shrink it says it can be reduced 1 GB space. Now when i do a backup of this database, does this space also getting added ...
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1k views

Schema for analytics table in Postgres

We use Postgres for analytics (star schema). Every few seconds we get reports on ~500 metrics types. The simplest schema would be: timestamp metric_type value 78930890 FOO ...
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2answers
217 views

Which is better? A join or a SELECT IN? [closed]

I have a set of ids that I extract using a join of 2 tables. Now I need some information from 2 other tables. What would normally be the best approach? So a SELECT IN using the set of ids I already ...
2
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1answer
95 views

How is the number of records on a select determined as high or low by optimizer?

What is the threshold between using the index and doing an direct access to the database? I assume this decision is related to the number of index access (and subsequent access to the tables). If this ...
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217 views

Optimizing multiple MySQL sub-queries and joins for multiple runs

The query (below) that's running is taking ~0.6 sec which normally would be fine but it gets run almost 1300 times for different custids. How can I make it run faster? Setup: Mac OS X 10.7.5 MySQL ...
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2k views

GUID stored in a varchar field

I have inherited several databases that use GUIDs as PKs. Instead of all the datatypes being uniqueidentifier most are varchar(50) and some varchar(100). Fields are true GUID some where created by ...
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39 views

Large-query optimization: given my “explain” results, what stands out?

I've got a monster query, which runs quite quick in many cases, but in some cases very much slower. The EXPLAIN output below is from one of the slow cases. I'm guessing that the slowdown is related ...
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0answers
1k views

Query optimization on a large table in PostgreSQL

I am not super experienced with databases, just have a college level course under my belt. I am trying to see if an existing query can be optimized. Currently on average takes over 300ms. There is a ...
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2answers
5k views

How to force higher values of open_files_limit or table_open_cache?

I'm having a frustrating problem where setting my.cnf to values higher than I currently have them does not 'take'. This is MySQL 5.6.14 on MacOS X 10.8.3 I'm trying to address dropped connections ...
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2answers
111 views

SQL Server 2000 optimzation job hasn't run for a while

Our SQL Server 2000 used to run a Sunday morning 3am job of optimizing the database. Somehow it was turned off and hasn't run in a year. We have noticed slow access to the database. We turned it ...
0
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1answer
251 views

SQL Server Index Scan Actual Executions

I have a fairly complicated query and running it on two databases with same schemas, but with different data produces different result. On database D1 the elapsed time is around 1 second, and the ...
3
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1answer
179 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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1answer
2k views

Optimize MySQL Self JOIN Query

I have a query which is running in 15+ seconds SELECT t1.`ST_StockCode`, t2.`SM_StockCode`, t2.`ST_ItemSize` FROM `stocks` AS t1, `stocks` AS t2 WHERE ...
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0answers
102 views

can this mysql query be optimized further?

I have this query select counterparty_id, max(id) max_id from client_administrations where counterparty_type = 1 and effective_on <= '2013-10-22' group by counterparty_id When I ...
2
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2answers
136 views

Convert to Self JOIN

I have a correlated sub query, which is taking 16 Minutes to execute. How can I convert it to join or optimize it. SELECT SSF.SM_StockCode, SSF.ST_ItemSize FROM Stock AS SSF WHERE ...
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2answers
305 views

How to optimize NOT IN statement

Why does NOT IN performs INDEX SCAN instead of INDEX SEEK? Here's my current index on the table, CREATE NONCLUSTERED INDEX [idx_dmcasarms_CourseList_cDesc] ON [dbo].[courselist] ( [c_desc] ...
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2answers
52 views

Convert to JOIN [duplicate]

I have a correlated sub query, which is taking 16 Minutes to execute how can i convert it to join or optimize it. SELECT SSF.SM_StockCode, SSF.ST_ItemSize FROM Stock AS SSF WHERE SSF.SM_StockCode = ...
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2answers
173 views

Filesort while using primary key for orderby

I have been breaking my head for the past two days now for this. CREATE TABLE `clients` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `guarantor_name` varchar(50) NOT NULL, ...
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2answers
206 views

How to optimize indexes on MySQL query with various sorts

I have an INNODB table levels: +--------------------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | ...
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3answers
185 views

Optimize extremely slow SQL query [closed]

SELECT j.job_id,j.last_updated,j.job_title,j.status, (SELECT COUNT(*) FROM candidates WHERE job_id=j.job_id) resumes_count, (SELECT COUNT(*) FROM candidates c WHERE job_id=j.job_id AND c.ref_type=1) ...
2
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1answer
1k views

How to optimize a query that includes counts of rows in multiple tables?

My query looks like this: EXPLAIN SELECT ... FROM USER u LEFT OUTER JOIN (SELECT COUNT(*) notified_count, user_id FROM user_email_tracking ON u.id = email_tracking WHERE email_type_id = 4 ...
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1answer
261 views

message table schema - body TEXT or varchar [closed]

I want to make a small social network. For messaging body which one is more suitable? VARCHAR or TEXT? if VARCHAR how long is efficient?