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

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7
votes
1answer
238 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
1answer
210 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, ...
1
vote
2answers
245 views

Does this cause overhead ? Select name from User where user_name LIKE '%alex%'

I'm wondering if this statement can cause real overhead if many concurrent users send the same request to mySQL InnoDB storage engine. Let's say I have a table that has the user_name, and someone want ...
2
votes
1answer
225 views

Why is the following query so bad performance-wise

After asking this question, I got to thinking about why a certain query was doing all the problems. In short, there was a query which took 500ms, I ran some execution plans and applied the ...
2
votes
3answers
214 views

How to get all non deleted messages from one user

I have two tables one for messages called default_messages and the other for recipients called default_recipient. This is how SQL for those table looks like: CREATE TABLE IF NOT EXISTS ...
0
votes
1answer
194 views

Normalization and Breaking down tables for InnoDB

I'm designing a web application, something small but deals with lots of read and inserts. Let's say 60% read and 40% writes. My storage engine is InnoDB on MySQL. I have a table called User which has ...
0
votes
3answers
3k views

Will more indexes on a table affect performance? [duplicate]

Possible Duplicate: How do indices impact query performance? How to know when/if I have too many indexes? Say ‘X’ report using some "where" condition in the statement so we are creating ...
7
votes
3answers
1k views

Is it better to populate variables using SET or SELECT?

What is the better way (with regards to performance) to set a value to variable? By SET command: DECLARE @VarString nvarchar(max); SET @VarString = 'john doe'; SELECT @VarString; By SELECT ...
8
votes
3answers
3k views

TSQL performance - JOIN on value BETWEEN min and max

I have two tables in which I store: an IP range - country lookup table a list of requests coming from different IPs The IPs were stored as bigints to improve lookup performance. This is the table ...
1
vote
2answers
159 views

Why are my results returned twice?

I've this query: SELECT DISTINCT `default_wall_posts_comments`.`c_id` as c_id, `default_wall_posts_comments`.`comments` as comments, UNIX_TIMESTAMP() - default_wall_posts_comments.date_created ...
1
vote
4answers
473 views

Performance degradation while updating tables having 10s of millions of records

I want to update tables ( my be 20-30 ) having 10s of millions of records each. The problem is that it is taking too much time for the update process and also at that time CPU usage also goes very ...
4
votes
1answer
3k 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 ...
1
vote
1answer
895 views

Index Strategies on Text or NVARCHAR(MAX) Fields

I have the following query (simplified for question) I'm trying to speed up for a read only DB ... SELECT [sysid] ,[Date]=CONVERT(CHAR, DATEADD(D, [date], '1800-12-28'),101) ,[From]=[from_addr] ...
1
vote
1answer
195 views

MySql - How can I improve this query?

I have the following table: http://pastebin.com/XH6FvhZ8 and the following query: SELECT * FROM (SELECT * FROM `job_logs` ORDER BY id DESC) AS job_logs_tmp WHERE (job_logs_tmp.user_id = ...
0
votes
1answer
139 views

Creating Indexes to optimize query

I have this query and I want to increase its performance by adding appropriate Indexes. DELETE FROM MYTAB1 WHERE MYID1 IN ( SELECT MYID2 FROM MYTAB2 ); I am not familiar with the ...
2
votes
2answers
2k views

Will altering order of columns in GROUP BY affect performance?

I have a SQL query like this: SELECT A, B, (CASE WHEN C=0 THEN 0 ELSE 1 END), COUNT(D) FROM SomeTable GROUP BY A, B, (CASE WHEN C=0 THEN 0 ELSE 1 END) On a huge dataset the actual execution plan ...
3
votes
2answers
251 views

How do I count rows with two properties in one index scan in SQL?

I have a table Units where I have rows ItemId, ItemCreationDate, ItemUnitsCount. ItemId is a primary key and I have a clustered index over it. I need to output the following: for each day I need to ...
6
votes
3answers
528 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 ...
4
votes
2answers
417 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; ...
2
votes
2answers
653 views

How expensive are SELECT FROM LIKE statements from a design perspective?

I have a situation running out of control regarding an ever increasing number of small static files I need to access randomly in a large directory structure. I MUST reduce the number of those files ...
2
votes
3answers
2k views

Is it normal to use many Triggers?

I have many so-called summary columns throughout my database to count number of rows in other tables. For example, counting the number of comments by a user (a column in the user table). Currently, I ...
3
votes
1answer
472 views

Measuring the relative performance of queries given the assertion that “cost” isn't reliable

The cost field is a comparative cost that is used internally to determine the best cost for particular plans. The costs of different statements are not really directly comparable. Source Does ...
1
vote
3answers
385 views

Checking if the query is made async or sync on SQL Server 2008 R2

I have tried to make async calls to my SQL Server instance as you may see on my SO post: ASP.NET MVC 4 asyncronous database call: never returns the HTTP response I did a benchmarking and the sync ...
6
votes
3answers
277 views

How can I identify when to create a new table to hold data that can be obtained from a query?

We have a payment table, and agents get commission on payments. Commission is based on a few different factors, such as how long it took to get the payment, so there is some calculations involved when ...
4
votes
1answer
4k 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 ...
4
votes
1answer
664 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 ...
3
votes
1answer
135 views

How to search for an ID and its descendants in this query?

I have the following tables: Project Unit ------------- --------------- ID | IDUnit ID | IdParent I have to select all projects that have the IDUnit ...
5
votes
2answers
932 views

Why does MySQL choose this execution plan?

I have two queries, select some_other_column from `table` order by primary_index_column asc limit 4000000, 10; and select some_other_column from `table` order by secondary_index_column asc ...
13
votes
3answers
3k views

Are Views optimized when I add a WHERE clause to them?

Does it make a difference if you filter a View inside or outside the View? For example, is there any difference between these two queries? SELECT Id FROM MyTable WHERE SomeColumn = 1 Or SELECT Id ...
3
votes
2answers
2k views

Copy huge amount from 1 table to other

I have two tables with same structure I need to copy over all tables from table A to table B, the problem is that some records from table A already exist in table B so that made the Import Fail. So I ...
9
votes
3answers
4k views

Why is an aggregate query significantly faster with a GROUP BY clause than without one?

I'm just curious why an aggregate query runs so much faster with a GROUP BY clause than without one. For example, this query takes almost 10 seconds to run SELECT MIN(CreatedDate) FROM MyTable WHERE ...
8
votes
1answer
1k views

Monumental difference in execution time between queries when using RECOMPILE query hint

I have two almost identical queries running on the same SQL Server 2005 instance: The first one is the original SELECT query as generated by LINQ (I know, I know... I'm not the application ...
5
votes
1answer
5k views

Why does a query run slower in a Stored Procedure than in the Query window?

I have a complex query which runs in 2 seconds in the query window, but about 5 minutes as a Stored Procedure. Why is it taking so much longer to run as a stored procedure? Here's what my query looks ...
3
votes
3answers
803 views

Speed up search across multiple columns

We have a table: user_info (user_id,domain_id,first_name,middle_name,last_name,work_place,.,.,.) Now, we have a user search that search across three columns first_name,last_name and work_place. ...
2
votes
4answers
957 views

Remove sub-string from WHERE clause

I have a stored procedure that is using the SUBSTRING function in the WHERE clause. SELECT DISTINCT ColumnBB, ColumnCC FROM TableAA WITH (NOLOCK) WHERE SUBSTRING(ColumnAA, 1, 17) = @VariableA; ...
4
votes
3answers
293 views

Which is the most efficient way to run a particular select query?

If I run the following code: select PolicyNumber, MAX(decpageid) as decpageid, Risk from StatRiskDecpages where PolicyNumber = 'AR-0000301132-04' group by PolicyNumber, Risk I get the following ...
5
votes
4answers
2k 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 ...
2
votes
1answer
232 views

Oracle ash report

Recently, i have been handed over with an ash report from DBA. To me this report is like french. I have no idea what it is about and what all is written in this report. Can someone please guide me in ...
1
vote
1answer
335 views

Oracle Benchmarking (SQL_NO_CACHE …)

I want to benchmark some queries on an oracle System from a JDBC application. However, the database seems to cache at some point in time. However, this is not intended in this case. Is there ...
3
votes
4answers
644 views

Help with Query. Finding records that have the same relationships (MySQL)

I have a table of Listings that has a many to many relationship with a Taxons table. The table structure looks like this: listings ---------------- id (int) name (varchar) listings_taxons ...
4
votes
2answers
466 views

Separate Address Table, Latitude, Longitude

We are designing a DB for a limited functionality social networking application. Users (Registered Users), Retail Outlets, etc are several entities we need to store. We need to store Address of each ...
0
votes
2answers
210 views

Attempting to avoid SORT in query planner

I have a query that is used VERY frequently in my application. Without modifying the query itself, I'm trying to optimize it by toying with different indexing schemes. Here's the query (basically): ...
2
votes
1answer
248 views

Query tuning - SQL Server

One of our developers are trying to run the below query on a development server, which involves pulling data from a linked server, production. The query ran for more than 14 hours before it was ...
3
votes
4answers
1k views

Optimize Query with Derived Table

I have the following query select ps.id, ps.title , IFNULL(s.likes,0) as num_likes , IFNULL(s.comments,0) as num_comments , IFNULL(s.ratings,0) as num_ratings , IFNULL(s.views,0) as num_views , ...
0
votes
1answer
134 views

Help me to better design my query so I can gain more performance

I need to find so called first entry in database for items regarding their partners. I wrote query and it is giving me correct results but it sucks with performances. I suspect that I have missed ...
2
votes
1answer
147 views

MySQL optimization question — how to speed up a simple query

I have a very simple database, one table with five columns (one INTEGER key, two VARCHAR, a DATE, and a DOUBLE). The table contains 909,000 records. A simple query like SELECT t.price FROM ...
0
votes
1answer
328 views

Returning Multiple Result-Sets or Single XML Blods

I've been writing some new Stored Procedures, and find myself returning between 6-8 result-sets in a few cases. I've always worked under the assumption that there were certain inefficiencies with ...
1
vote
2answers
125 views

Improving MySQL efficiency

I have a table now containing over 43 million records. To execute SELECT, I usually select records with the same field, say A. Will it be more efficient to divide the table into several tables by ...
8
votes
1answer
491 views

How to get accurate query performance?

I'm attempting to improve performance of a stored procedure. When I run the SP, it finishes almost instantly, as if something were cached. I was told to use the following two lines of SQL before ...
3
votes
1answer
514 views

Can you have a WHERE clause with a greater than operator on a composite key?

I'm a web developer (not DB admin!) having performance problems processing a 1.7m record table on MySQL. Here is that table : CREATE TABLE `bbcmapimp_pc_raw` ( `postcode` char(8) NOT NULL, ...