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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Specifying columns in LEFT JOIN through subquery beneficial performance wise?

I'm trying to understand how JOINS are handled, and was wondering about the following: Imagine a big query concerning a lot of tables, I'm really looking for small optimizations as it's a pretty ...
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328 views

How do I perform a batch update on the first n matching rows?

I need to perform a large number of updates at the same time. Each update needs to only affect up to a specific number of rows even though the WHERE clause might match to many more rows. So far, I'm ...
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1answer
55 views

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
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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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1answer
501 views

Is there any way to save the execution plan in a file in PostgreSQL?

By saving a plan in a file, I don't mean to view it, but force the optimizer to use the saved plan to execute some query. For example, if the optimizer chooses to do a hash join for a two-table ...
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311 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
3k 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, ...
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1answer
374 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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1answer
67 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
642 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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1answer
5k views

Why does SQL Server run a subquery for each row of the table it's qualifying?

This query runs in ~21 seconds (execution plan): select a.month , count(*) from SubqueryTest a where a.year = (select max(b.year) from SubqueryTest b) group by a.month When the subquery ...
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5k 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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755 views

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 ...
2
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1answer
46 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 ...
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2answers
917 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
1k 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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3answers
1k views

Oracle is not using a unique index for a long key

I have a table with 250K rows in my test database. (There are a few hundred millions in production, we can observe the same issue there.) The table has an nvarchar2(50) string identifier, not null, ...
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1answer
511 views

Where does mysql write temporary tables?

MySQL 5.1.x | Windows Server 2008 Data Directory is on a different partition. I have an innodb database (innodb_file_per_table is set) that I'm trying to optimize and I don't appear to have enough ...
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1answer
325 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
9k views

How to optimize InnoDB tables in MySQL

I’ve been researching how to optimize only fragmented tables in MySQL and reviewed this post on optimizing tables. It basically performs a query against the information_schema database for any table ...
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1answer
103 views

Is it possible to optimize this query? Or any recommendations to speed it up?

After looking at my mysql slow log file I see that almost every entry there is related to this query: SELECT n.nid AS nid, AVG(v.value) AS rating FROM node n LEFT OUTER JOIN votingapi_vote ...
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1answer
9k 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
95 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
201 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 ...
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1answer
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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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94 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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699 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
103 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
109 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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590 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
201 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
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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 ...
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41 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 ...
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350 views

Oracle 11g INDEX_SS Hint

Given a table with columns A, B, and C of type CHAR(15) and an unique index If I have a query with a WHERE clause specifying A and C, oracle's optimizer chooses to do an index range scan. But if I ...
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1answer
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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
108 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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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
297 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
363 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 ...
5
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1answer
2k 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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227 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
106 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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1answer
235 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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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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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
6k 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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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 ...
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1answer
350 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 ...
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1answer
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 ...