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Questions tagged [optimization]

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 there a way to hint to query optimizer to MySQL which constraints should be done first?

This is my current query: SELECT BusinessID as ID, 111151.29341326*SQRT(pow(-6.186751-X(LatLong),2)+pow(106.772835-Y(LatLong),2)*0.98838574205337) AS Distance from ( SELECT * FROM ...
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4answers
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MySQL: Index when joining to tables not being used (Performance optimizing question)

I need to optimize the following query: SELECT /* [things omitted] */ articles.blogpost_id, articles.id AS articleid FROM blogposts JOIN articles ON articles.blogpost_id = blogposts.id WHERE ...
6
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2answers
126 views

Will the Oracle optimizer still plan a join if I'm not reading any columns in the joined table?

Suppose I have the following view: CREATE VIEW dummy AS SELECT a.some_column, b.other_column FROM a LEFT JOIN b USING (key_column); When I do a SELECT some_column FROM dummy, is there a way to ...
6
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1answer
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How to tune the query on all_constraints in Oracle 12c?

How to tune the following query, as it's taking around 8 seconds: select constraint_name,table_name from all_constraints where r_constraint_name in ...
6
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1answer
161 views

Are there any fully declarative implementations of SQL

I was recently bitten by a bug ( in my code ) where these two queries had drastically different runtimes: select * from smalltable st inner join bigtable bt on st.btid = bt.btid select * from ...
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4answers
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Optimizing a compare query

Suppose I have two tables, A and B, and I know that size(A) = size(B). I want to confirm that the data in both tables is the same in three given columns, suppose they are X, Y, and Z (there are no ...
6
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2answers
585 views

Will the Query Optimizer Ignore A Fragmented Index?

Scenario: I have a heavy OLTP table with an index. I see many inserts, updates and deletes and the index fragments heavily within a day or less. While on day one of the index build, the optimizer ...
6
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2answers
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Improving `GROUP BY` query performance in sqlite3

I have a little web-application that is using sqlite3 as it's DB (the db is fairly small). Right now, I am generating some content to display using the following query: SELECT dbId, dlState, ...
6
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1answer
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Password hashes: Fixed-length binary fields or single string field?

I'm currently having am amusing debate with a friend, and we simply can't agree on the best method to store salted passwords in a database. The two options on the table are: Storing the hash and the ...
6
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1answer
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Should i use subquery to limit table before a join?

In the case of a join followed by a where clause, would it be better to use a subquery to limit the results, and then do the join? Example: SELECT * FROM Customers NATURAL JOIN Orders WHERE ...
6
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1answer
651 views

SQL Server Plan Guide

I have a query that needs a plan guide, but I am having a hard time setting it up. Query below from the procedure cache... (@state nvarchar(14), @jobName nvarchar(18), @jobGroup nvarchar(28), @...
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1answer
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Why does `SELECT count(*)` return 1?

Some RDBMS's seem to allow a query without a FROM clause, eg: postgres=# SELECT 'foo' bar; ┌─────┐ │ bar │ ├─────┤ │ foo │ └─────┘ But for those that allow it, why would SELECT count(*) not return 0?...
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3answers
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Optimizing a simple query on a large table

DB: MySQL 5.5.20 (WampServer, default configuration) OS: Win 7 HDD: Western Digital 3TB Caviar Green, 3.5", IntelliPower, 64MB, Sata3 (WD30EZRX) Memory: 8GB MySQL my.ini: http://pastie.org/private/...
6
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1answer
200 views

Why doesn't first execution plan use RowCount Spool?

Here is my repro that was tested on SQL Server 2008 R2, 2012, 2016. The second and the third queries do use RowCount Spool, why the first does not? create table dbo.t (id int identity primary key, v ...
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2answers
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Optimizing “WHERE x BETWEEN a AND b GROUP BY y” query

CREATE TABLE test_table ( id uuid NOT NULL, "RefId" uuid NOT NULL, "timestampCol" timestamp without time zone NOT NULL, "bigint1" bigint NOT NULL, "bigint2" bigint NOT NULL, "int1" integer ...
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1answer
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Stored procedure with recompile

I have this stored procedure..when i told the developers that it is not recommended to use with Recompile option they replied "This was because this SP can be called with a number of different params ...
6
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2answers
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Optimizing mysql queries running on 50 million rows

Is there any technique/advice in order to optimize the performance of the queries below? According to my needs, my average db size will be approximately 30mill. rows per day, so every second less, ...
6
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2answers
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Which is faster? Multiple update queries in a cursor loop, or a single query?

Which is faster? Multiple update queries in a cursor loop, or a single query? For example, in this case, which is faster? This query is in a trigger. Multiple Query in loop DECLARE done INT DEFAULT ...
6
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2answers
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TokuDB optimize table does not reclaim disk space

Was running an optimize table command against one of my tables about 11Gig in size, surprisingly used disk space grew by 2Gig immediately after optimization process finished. I would expect the ...
6
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1answer
188 views

Ok to drop indexes on FK's if they have no stats in the DMVS

I have used the well known query below from Kevin Kline to check for unused indexes. Several indexes created on Foreign keys returns no read stats, only writes. Are you 100% safe to drop these ...
6
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1answer
220 views

Help with interpretation of execution plan (Hash Aggregation)

So I have an execution plan I'm trying to optimise, and there's a section of an operator I can't quite understand. There is a section of the plan where a Compute Scalar is used, followed by an ...
6
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1answer
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Running out of disk space running MySQL query

I have been trying to create a "gaps and islands" identification routine in MySQL. See the previous question regarding setup here and computational resources required here. I increased the RAM of the ...
6
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1answer
3k views

How much does table size affect performance?

Let's say I have a MyISAM table (amongst other tables) with 2938347 rows. This table is merely being used for logging and doesn't get emptied very often. By not regularly archiving or deleting these ...
6
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1answer
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Optimizing MySQL for Read-Only?

We have 14 GB worth of CSV's which total 138 million rows. I imported this into a MySQL table first with InnoDB, and then tried again with MyISAM. In both cases, a simple SELECT on primary key (...
6
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1answer
552 views

Joining in query decomposition

I have a question about query decomposition and localization, as in here: When joining 3 tables (here EMP, ASG and PROJ) is there any specifiy order, when to join which table? Why ASG and EMP are ...
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1answer
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New query slower even if it has less logical reads than the old one

I have a query in a stored procedure that must list all dates in a range and JOIN on it date from a table, if it exists for that day. The procedure was created before I was employed as the DBA, so I ...
6
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1answer
656 views

Determining which isolation level is appropriate

This is a homework question. For the following transactions state the isolation level that will maximize throughput without lowering the integrity of the database. Explain the answer. ...
6
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1answer
6k views

PostgreSQL tree structure and recursive CTE optimization

I'm trying to represent a tree structure in PostgreSQL (8.4) to be able to query the path from the root to a given node or to find all the nodes within a sub-branch. Here is a test table: CREATE ...
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2answers
990 views

How to use merge hints to isolate complex queries in SQL Server

I can understand that if I join individual queries that are "individually fast" the combination may become slow because the default execution plan may be non-optimal. However when I know the number ...
6
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2answers
120 views

Why do we have Top N Sort in this example?

In the example below the result of Index Spool is already sorted, why do we have Top N Sort here instead of simple Top? use tempdb; go create table dbo.t1 (id int identity primary key, v int); create ...
6
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1answer
114 views

Can oscillating number of fragment decrease FTS query performance?

I've got a FTS Catalog on a table containing ~106 row. It used to work like a charm, but lately, for an unknown reason it started to get very bad (query >30 sec) performances randomly. Reading ...
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1answer
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Optimized MySQL “optional” FULLTEXT search on multiple tables

I need to select data with optional columns in where clause based on client input. my query is like this: SELECT a.id, a.title, b.txt AS b_txt, c.txt AS c_txt FROM a LEFT JOIN b ON a.b_id=b.id ...
6
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1answer
276 views

Demystification of SQL Server optimization process [duplicate]

We would like to see all variants of query plan considered during a query optimization by a SQL Server optimizer. SQL Server offers quite detailed insight using querytraceon options. For example ...
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0answers
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Slow Postgresql query with left join

I have a query on Postgres, and I also have added proper index. Is there anything missing here? SELECT orders.*, demo.name as d_name FROM orders LEFT JOIN users as demo ON demo.id = orders.dr_id ...
6
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1answer
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Query performance while using sqlcmd [duplicate]

Possible Duplicate: Running queries with SQLCMD vs. Running queries with SSMS I have a number of scripts that query the data from different tables and consequently update them. I have defined a ...
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3answers
6k views

Optimize UNION query in MySQL

I have a problem with a UNION query in MySQL. We have 10 millions players on our website and we would like to select players with a multi-criterias system. For exemple, selecting US people, men, more ...
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3answers
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Union implemented with Hash Match operator

I was reviewing the SQL Server physical operators listed on TechNet (don't judge, you know you've done it) and read that the Hash Match physical operator is sometimes used to implement the UNION ...
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1answer
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Is the NOLOCK hint completely safe to use on a table that's guaranteed not to change?

If I have a table where the data is guaranteed to never change, is it completely safe to add the NOLOCK hint to my SELECT queries against that table? If I have a table where some of the rows will be ...
5
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1answer
485 views

How to get all records from one table and see if they are used in another without a sub query

I'm trying to write a SQL statement that pulls down all the records from one table and looks to see if they are used in another. SQL is not my strongest language, so I'm sure this is a pretty simple ...
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2answers
12k views

How to lower “Created_tmp_disk_tables” - temporary tables count created on DISK?

I have a ubuntu, mysql 5.6 DB that is heavy used by thousands of users. After 10 days of running I have this values: | Created_tmp_disk_tables | 894170 | | Created_tmp_files | 26068 | | ...
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3answers
925 views

Does the 'quality' of query optimization differ by database provider? [closed]

I don't have a clear understanding of query optimization in any database, other than that 'it happens'. Now I've just seen a "medium" post that discusses the pitfalls of using a PostgreSQL CTE in ...
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3answers
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Does a query with a primary key and foreign keys run faster than a query with just primary keys?

SELECT something FROM table WHERE primary_key = ? vs. SELECT something FROM table WHERE primary_key = ? AND other_key = ? Say that this is a scenario where the inclusion of other_key does NOT ...
5
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1answer
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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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4answers
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MySQL Query Optimization : Indexing and Pagination

I'm adding pagination to my 'latest news' php script and running in to an issue. SELECT sid, title, time, bodytext, author, url FROM news WHERE approved=1 ORDER BY time desc LIMIT $start, $limit I ...
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3answers
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Order of composite index

Suppose I have a query like: SELECT * FROM table_a JOIN table_b USING (id) WHERE table_b.column = 1 I have an index on id and an index on column but often I add a composite index with both that ...
5
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1answer
543 views

Optimize where clause using dates

I'm trying to optimize several queries that all use a similar pattern on one of the WHERE clauses: AND (DATEADD(DAY , ISNULL(a.[due_days], 30) + 30 , [dbo].[CalcDate]([type], ...
5
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2answers
28k views

forcing Oracle to use hash join for a subquery

I have a query that looks like SELECT * FROM table0 WHERE id IN (SELECT id FROM table1 JOIN table2) Oracle is choosing to join table0 with the result of (table1 x table2) using nested loops and ...
5
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3answers
1k views

Does SQL server optimize or pre-parse stored procedures?

Despite what DBAs will tell you ("use stored procedures because the server will optimize them thus they are faster than ad-hoc queries"), I have heard that SQL server does not actually optimize stored ...
5
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1answer
793 views

Scalar function value not being stored

I created a scalar valued function that pauses for as many seconds as passed in. This function won't be used in production. I am trying to test how SQL server evaluates and creates plans if I add a ...
5
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1answer
12k views

Why isn't oracle using an index for distinct query ?

This is the scenario SQL> exec dbms_stats.gather_table_stats(user,'TM', cascade=>true) PL/SQL procedure successfully completed. SQL> SELECT SEGMENT_NAME , SEGMENT_TYPE , BYTES / 1024 / ...

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