Tagged Questions

In the context of a database, optimisation refers to the process of the query optimiser selecting the best query plan to execute.

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MySQL: Large data slow reads

I have very large table with 17,044,833 Rows and 6.4 GB in size. I am running the simple query below and it takes like 5 seconds. Any ideas what optimizations can I do to improve the speed of this ...
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84 views

Opimizing Mysql

Below is a Snapshot of my Server info. And the results of mysqltuner.pl Some help with commands would be appreciated. I'm new to this and have gotten pretty far, but I'm having issue with Mysql ...
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60 views

Why is adding a FK constraint slow in MySQL?

I'm doing some experimentation in MySQL 5.1.53 and I've run across a case that I just don't understand. CREATE TABLE A ( `pk` INT(11) NOT NULL, `fk` INT(11) NOT NULL, PRIMARY KEY `pk_A` ...
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1answer
73 views

Will a specific denormalization increase performance in a specific scenario?

Suppose I have a table with fields | a | b | c | d | ... | z | which contains millions of records and which will serve millions of queries a day. And the field z holds the id of some parameter which ...
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1answer
143 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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1answer
286 views

mysql optimize group by having count

I have 2 mysql table: table t1: CREATE TABLE IF NOT EXISTS `t1` ( `id` bigint(20) NOT NULL, `url` varchar(255) DEFAULT NULL, `title` varchar(255) DEFAULT NULL, `type` varchar(255) DEFAULT ...
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3answers
700 views

Indexes for SQL query with WHERE condition and GROUP BY

I am trying to determine which indexes to use for an SQL query with a WHERE condition and a GROUP BY which is currently running very slow. My query: SELECT group_id FROM counter WHERE ts between ...
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2answers
160 views

optimization of expensive subquery

The following query gets the last report (where latitude and longitude and secs are <> 0) associated with the specified units: SELECT reports.* FROM reports INNER JOIN ...
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2answers
64 views

index for OR operator with different columns

What would be best index for this kind of query: -- id is the PK create table test3(id number,name varchar2(30) not null,lastname varchar2(30) not null); select id from test3 where id=:1 and ...
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1answer
65 views

Can this where in subquery be optimized?

Currently the query takes ~ 0.2 seconds. Is this the best it gets? Because it feels rather slow for the amount of data I have in my database. I've tried creating an inner join out of it but I can't ...
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2answers
578 views

How can I speed up this query on table with 65 million records

select hash_id, hash_value, substring(T.cri, 1, 7), substring(T.cri, 9, D2.Pos-9), substring(T.cri, D2.Pos + 1, D3.Pos - D2.Pos - 1), ...
1
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1answer
204 views

Can I rely on this SQL shortcut to save a query?

Consider a table with a status field that holds one of two values: | status | set('active','default') | YES | | active | I need to know how many records are in the table, and additionally how ...
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0answers
30 views

Optimization for a dynamic M2M value field

I'm using a Configurable Product methodology in my system. Everything works great, but I have one thing I think that might strongly impact the performance in the future. This is a dynamic linking ...
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1answer
214 views

MySQL hangs in killed state

I have a strange situation where a mysql OPTIMIZE query will never end. After killinh it, it remains in the "killed" state. Scenario to reproduce: OS: CentOS 6.5 MySQL: 5.6 Engine: MyISAM (with at ...
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0answers
67 views

query comes in slow query when log_queries_not_using_indexes is enabled

This query comes in slow query log when log_queries_not_using_indexes is enabled. SELECT a.id, d.id AS city_id, d.city FROM booking a, sub_area_master b, area_master c, city_master d WHERE ...
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0answers
98 views

expdp taking so much time

I'm doing a full export using the expdp tool. I'm using 4 dump files and parallel parameter with 4 value. The export size calculus shows 30Gb but it takes almost 4 hours for 4 Gb. I'm pretty confused ...
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1answer
56 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
84 views

Optimising an Update statemement with a very large Where clause

I have a table (TableOne) with a field called [ReviewNote], this table has more than 1.5 million record, I have to assign keywords to each record based on the content of the [ReviewNote], using ...
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0answers
32 views

Which of these two queries is better - can be optimized more?

With regards to the usage of the IF statement, which is better to use? Query #1: SELECT whatever FROM auctions AS a LEFT JOIN ............... WHERE ............... ORDER BY ...
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1answer
223 views

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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0answers
93 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 ...
5
votes
1answer
50 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
342 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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0answers
166 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
910 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
231 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
59 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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0answers
75 views

Is this a normal query time for a left outer join type?

I have Employee Table - 80K records about 100 fields I have an app table - 8K records and 30 fields Then a subset of employee table also stored as table that has our divisions subset - call this ...
2
votes
1answer
231 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
1k 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 ...
2
votes
1answer
40 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 ...
2
votes
2answers
392 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 ...
0
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1answer
392 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
130 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 ) ...
3
votes
1answer
3k 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
79 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
124 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
93 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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0answers
83 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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0answers
199 views

how I can set tmp_table_size for 16GB long table?

I am posting the same question as I posted in SO, but somebody advice me to post it here. I have a 16GB big table with 8 partition. And RAM 4 GB. I am running a query - SELECT ...
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0answers
45 views

Tips for improving my particular batch processing

I'm working with mail data from the USPS, which contains 10 related tables to make up a single mailing, as well as a single table containing all of the scans that each mail piece may receive. Once per ...
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1answer
257 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
73 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 ...
0
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1answer
96 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
70 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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0answers
43 views

How to make a process use all the power of the machine (MySQL process)

I need to do a serious migration of data from MySQL to MongoDB. I have dumped the actual database from its server to the Amazon AWS box. I set up a powerful Amazon AWS Ubuntu EC2 12.04.3 type ...
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1answer
138 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
57 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
37 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
211 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 ...