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Optimising and caching your MySQL enquiries

If you want to get a list of all the different values in a column, but don't care how many times each occurs, you can apply the DISTINCT keyword on your SELECT. See [here]. The DISTINCTROW is a synonym of DISTINCT, and the default is ALL which you may also state.

MySQL Caches queries - so that if it receives an identical query for a second time, it doesn't have to do all the hard work again. MySQL is aware of changes that may be made to the tables, so that it can identify when the results would be changed and so it needs to re'calculate', so most of the time you can simply say "Thank you" for the facility and write code that repeats the same query many times over, secure in the knowledge that MySQL will spot your inefficiencies (it does this across sessions too - see [here]). There are, however, times that you don't want caching to be running - the most common example is if you're benchmarking your application and you want to see if a change that you've made has made things faster or slower; in such a situation, cached results will cause major variations in the times you get which are not related to the software changes that you're making. You can apply the SQL_NO_CACHE keyword to your select statement to ensure that the results you generate are not saved in the cache ... so that subsequent requests will not refer back to this results set. [[You can set SQL_CACHE if you explicitly want caching on; it's the default]]

Aside - although MySQL will help you by using the cache when it's sure that it can, you should not rely on it as a crutch for your own sloppy, repeated coding. It will err on the side of caution and recalculate if there's any possible doubt, and when you think of it, that's a far better solution than risking wrong results!


Other keywords that you can apply directly after the SELECT are:

HIGH_PRIORITY - Do the SELECT in preference to any outstanding updates

STRAIGHT_JOIN - For optimizing joins (to help the built in optimiser)

SQL_SMALL_RESULT, SQL_BIG_RESULT, SQL_BUFFER_RESULT - To help MySQL optimise its performance based on the anticipated size of the result set

SQL_CALC_FOUND_ROWS - If you're doing a query and only asking for some of the results back through LIMIT, this keyword will tell MySQL go go ahead and find ALL the results not just the few you want, in so far as it will be able to tell you how big the result set would have been without the LIMIT.

The extra keywords are MySQL extensions to the SQL standard, and if you use them you'll restrict the portability of your code (that may not be a problem to you ... but it's worth bearing in mind if you may use a different database engine in the future!)

(written 2010-02-22, updated 2010-03-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
S157 - More MySQL commands
  [4481] Extracting data from backups to restore selected rows from MySQL tables - (2015-05-01)
  [3270] SQL - Data v Metadata, and the various stages of data selection - (2011-04-29)
  [3061] Databases - why data is split into separate tables, and how to join them - (2010-11-20)
  [2647] Removing duplicates from a MySQL table - (2010-02-22)
  [2644] Counting rows in joined MySQL tables - (2010-02-22)
  [2643] Relating tables with joins in MySQL - (2010-02-21)
  [2448] MySQL - efficiency and other topics - (2009-10-10)
  [2259] Grouping rows for a summary report - MySQL and PHP - (2009-06-27)
  [2110] MySQL - looking for records in one table that do NOT correspond to records in another table - (2009-03-31)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1574] Joining MySQL tables revisited - finding nonmatching records, etc - (2008-03-15)
  [1331] MySQL joins revisited - (2007-09-03)
  [1235] Outputting numbers as words - MySQL with Perl or PHP - (2007-06-17)
  [1213] MySQL - the order of clauses and the order of actions - (2007-06-01)
  [673] Helicopter views and tartans - (2006-04-06)
  [591] Key facts - SQL and MySQL - (2006-02-04)
  [581] Saving a MySQL query results to your local disc for Excel - (2006-01-29)
  [572] Giving the researcher power over database analysis - (2006-01-22)
  [567] Combining similar rows from a MySQL database select - (2006-01-17)
  [517] An occasional chance, and reducing data to manageable levels - (2005-12-04)
  [515] MySQL - an FAQ - (2005-12-03)
  [513] MySQL - JOIN or WHERE to link tables correctly? - (2005-12-01)
  [502] SELECT in MySQL - choosing the rows you want - (2005-11-22)
  [494] MySQL - a score of things to remember - (2005-11-12)
  [449] Matching in MySQL - (2005-09-24)
  [279] Getting a list of unique values from a MySQL column - (2005-04-14)
  [159] MySQL - Optimising Selects - (2004-12-21)
  [158] MySQL - LEFT JOIN and RIGHT JOIN, INNER JOIN and OUTER JOIN - (2004-12-20)


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