Tagged Questions

A class of issues where a database executes a query inefficiently due to the optimiser selecting an innapropriate query plan.

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2
votes
2answers
314 views

MySQL index on column comparison

How do you go about setting an index to be used for a column comparison like WHERE col1 > col2? col1 and col2 are of DATETIME type.
11
votes
3answers
1k views

Improve performance of query using IN()

I have the following SQL query: SELECT Event.ID, Event.IATA, Device.Name, EventType.Description, Event.Data1, Event.Data2 Event.PLCTimeStamp, Event.EventTypeID FROM Event INNER JOIN ...
0
votes
1answer
303 views

Design database to query a date range for scheduling

I am designing a db that should include a table with fields for datetime scheduling, so as I know what kind of queries will be made​​ periodically I would like to design the table for them to be as ...
1
vote
1answer
706 views

Splitting a column to normalized table

I have a de-normalized table as Original De-Normalized Table id text 23 first,second|third,fourth,fifth|sixth I want to normalize the database to create the following tables Table 1: (Split by ...
1
vote
1answer
253 views

Simplify and optimize a complex query

Our site has a chronological event feed that uses MySQL. We have noticed that some of the queries tend to run for quite a bit longer than others. For example, some queries for our heaviest users take ...
2
votes
1answer
1k views

how to improve query performance

## Only 1 query which returns 10 rows. ## It took about 66 secs mysql> explain SELECT al.log_id , al.user_id , al.page_name , al.section_name , al.submit_time , al.url , ...
2
votes
3answers
299 views

mariadb ignoring force index

I'm trying to tune a relatively simple query: select bt.col1,bt.col2 from bigtable bt join smalltable st on bt.smalltable_id = st.smalltable_id where st.name = 'some name occuring only once in st' ...
1
vote
1answer
219 views

mysql freeze/crash on JOIN

here is query: SELECT nw.news_id, nw.title, nw.description, nw.url, nw.date, nw.image, nw.status, src.title as src_title, src.keyword as src_keyword, src.url as src_url, src.icon as ...
0
votes
2answers
183 views

How can I optimize this query and support multiple SKUs?

My current query only can select one SKU at a time. I can leave salesite_id constant. If there is a way to also have varying salesite_ids that would be good too, but not necessary. Also any ...
4
votes
2answers
5k 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 ...
2
votes
2answers
3k views

How do I force a JOIN to use a specific index in MySQL?

I have a query that JOINs 2 tables; lineitem and part, select sum(l_extendedprice* (1 - l_discount)) as revenue from lineitem force index for join (l_pk), part where ( ...
1
vote
1answer
105 views

Topics and posts represented by one table or two?

I'm currently building a forum component for a larger application and I'm considering different approaches to certain parts of the database schema. In particular, I am considering representing topics ...
0
votes
2answers
599 views

Slow query with straight_join for small result

I have a performance-problem with a query in my application and I don’t understand the behavior of mysql. The query consists of different joins, especially the join to the mentioncache-table. If the ...
10
votes
1answer
5k views

SQL query for combinations without repetition

I need a query which can be used in (or as) a function and retrieves all combinations of n values. And I need all combinations of length k where k = 1..n. Extended sample input and result so input ...
2
votes
2answers
290 views

Can I use a foreign key index as a shortcut to getting a row count in an INNODB table?

I have a table that has a large number of rows in it. The primary key (an auto-incrementing integer) is, by default, indexed. While waiting for a row count to be returned I did an EXPLAIN in another ...
-3
votes
2answers
912 views

Optimisation sub query SQL Server

I would like to optimise the query below because it's actually taking 40% of the time in the real one. I HAVE to get the first line in the table 2 for each line in the table 1. The relationship is 1 ...
2
votes
0answers
119 views

How To Analyse Index Scans

I am using merge replication with SQL Server 2012. During a sync using a merge agent there are a large number of index scans. I can see this by running a profile capturing SCAN:Started and ...
2
votes
1answer
729 views

Oracle 10g using collection with sql

I have a stored procedure that takes collection of objects. (TABLE OF MyCustomType). Inside the procedure I'm trying to join this parameter with real tables. For example, something like create or ...
2
votes
1answer
550 views

inner join on PK with extra criteria slow despite indices?

Given the two tables below I am struggling to understand: why is the third query slow even though first two queries are fast what exactly is EXPLAIN saying can I do anything to significantly speed ...
3
votes
2answers
74 views

Looking for a possible multi-column index

I have a table highscore containing the columns: game (text) date (timestamp) score (integer) more irrelevant ones... The query most often run on it is: SELECT * FROM highscore WHERE game = ...
3
votes
2answers
529 views

Do I risk losing the benefits of indexing if I have an index on every column?

I use PostgreSQL 9.2. I have a table with ~5 million rows and 150 columns. The table does not change at all (I replace it once a year). Users query this table with all kinds of filters on any some of ...
14
votes
2answers
13k views

Increasing work_mem and shared_buffers on Postgres 9.2 significantly slows down queries

I have a PostgreSQL 9.2 instance running on RHEL 6.3, 8-core machine with 16GB of RAM. The server is dedicated to this database. Given that the default postgresql.conf is quite conservative regarding ...
5
votes
2answers
3k views

What can I do to speed up this SQL Query?

I've created this SQL query with the help of @Dems :-) Here is some detail, I tried to make a SQLFiddle but I kept getting errors with my variables... This works in Sql Server 2008... My question ...
2
votes
2answers
730 views

Why does my query run faster after I force full table scan?

I have a MERGE statement that took forever to execute the select as the table it is running from (SUB_DATA) is rather big (read 1.4 Tb, 700 million rows). My SELECT looks like this. SELECT ...
1
vote
1answer
1k views

Query performance of a NULL vs an empty ('') varchar in SQL Server 2012

I have 3 indexed fields in a query: int, int, and varchar(250). The query performs well when all 3 conditions are specified with real values. The int columns always have values, but there are plenty ...
2
votes
1answer
591 views

oracle merge operation hangs (or loops); query tuner claims it's a cartesian product operation

I have a Merge query (open to better suggestions) that seems to cause the server to hang. (I'm not sure what it's doing, but it pegs the CPU and never completes) When I run it through the query ...
1
vote
1answer
345 views

Optimizing User Defined Custom Columns

I am building a web based app that has the potential to have very high traffic and collect a lot of data. Most of this traffic will be utilizing the database. Database is MySQL InnoDB. I want to give ...
6
votes
4answers
3k views

What can speed up a SQL count query?

When doing a count (aggregate) SQL query, what can speed up the execution time in these 3 database systems? I'm sure many things could speed it up (hardware for one), but I'm just a novice DBA, so ...
1
vote
1answer
331 views

Query tuning help needed (Hash Match and Table Scans)

I need help optimizing the following query (returning ~8k rows): SELECT A.sys_id, 'AppSvrRels' = CAST(SUBSTRING((SELECT (', ' + T.name) FROM ( ...
5
votes
2answers
1k 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 ...
9
votes
3answers
437 views

Are these two queries logically equivalent?

Are these two queries logically equivalent? DECLARE @DateTime DATETIME = GETDATE() Query 1 SELECT * FROM MyTable WHERE Datediff(DAY, LogInsertTime, @DateTime) > 7 Query 2 SELECT * FROM ...
2
votes
1answer
134 views

Combining columns in index

With reference to RolandoMySQLDBA's answer on MySQL: Index when joining to tables not being used (Performance optimizing question) He mentioned Make sure you build an index that involves the ...
3
votes
3answers
2k views

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 ...
1
vote
1answer
1k views

Slow complex query with group/order

I have a problem with a very important query in my application. It is very slow when adding group and order to it. So the full query is: SELECT m.id, m.title, m.title_text, ...
1
vote
2answers
825 views

Slow mysql fulltext query

I have a little problem with the performance of a mysql-query which uses a fulltext index. The following query SELECT Mention.id FROM mentions AS Mention WHERE (MATCH (`Mention`.`title_text`, ...
1
vote
1answer
187 views

What would be a optimal index for this query in mysql

I'm trying to optimize this query. SELECT * FROM sales_reps WHERE account_id = 1 AND (state = 'enabled') AND (clients >= 5 OR revenue >= 10000) ORDER BY name, platform, client_group, ...
2
votes
1answer
98 views

Optimising query on view that merges similar tables with a clear discriminator

Using PostgreSQL 8.4, I have a number of tables that have a very similar structure, but that belong to different categories: CREATE TABLE table_a ( id SERIAL PRIMARY KEY, event_time TIMESTAMP ...
3
votes
3answers
349 views

Why does the query cost change so much and how to prevent it

The query causing me the issue is too big, but the piece that seems to be the core is quite simple, I will try to go with that: The query has an structure like: SELECT ... a lot of stuff ... ...
1
vote
1answer
94 views

What is the best relationship model for high performance?

I want to model a relationship between Customer and Transaction. However, because Customer is only described by a unique string and no extra info, I have two possibilities to implement it in the ...
6
votes
1answer
171 views

How should we handle rows which won't be queried once they are old in PostgreSQL?

We have a table in a PostgreSQL database which is growing on the order of millions of rows per day. Each row consists of: ID Foreign user ID Date and time Other data The date and time aren't ...
4
votes
1answer
137 views

help required with indexing and query optimzation

After browsing most of the posts and after running out for options I am posting here. I have a table with 76,222,954 (76M) rows. It collects web site traffic based on country. Here is my table ...
1
vote
2answers
110 views

How can I improve this query

I have the following table: CREATE TABLE `twitter_relationships` ( `id` int(11) NOT NULL AUTO_INCREMENT, `user_id` int(11) NOT NULL, `source_twitter_id` bigint(20) NOT NULL, ...
3
votes
2answers
203 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 ...
12
votes
2answers
3k views

How (and why) does TOP impact an execution plan?

For a moderately complex query I am trying to optimize, I noticed that removing the TOP n clause changes the execution plan. I would have guessed that when a query includes TOP n the database engine ...
2
votes
1answer
126 views

How to know what to investigate and how for a VERY slow stored procedure

I have inherited a large and slow stored procedure and it's giving me a nightmare: I'm not a DBA, although I have some knowledge, but I don't know where to start to identify this bottleneck. I have ...
3
votes
1answer
7k 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 ...
3
votes
2answers
270 views

Do MySQL sub-queries essentially use as much overhead as separate queries?

If I have a query like this: SELECT name FROM ( SELECT * FROM `table` WHERE id IN (1,40,300) ORDER BY AnotherColumn ) From a bandwidth and overhead standpoint, is that the same thing as doing ...
1
vote
1answer
221 views

query optimization delete nested query [closed]

Can the following query be optimized ? DELETE FROM COLOR_TAGS WHERE COLOR_PROJECTID = $id AND NoteID IN( SELECT ID FROM note_note WHERE BibID=$bid AND isdel=0);
0
votes
0answers
165 views

How to load XML data into an oracle db table ?

Is there a way to load the XML format data into an Oracle 11g DB table directly ? I mean, Can we write a Stored Procedure to accomplish the same in a more efficient way with the best performance ? ...
4
votes
2answers
3k 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 ...