Comments on: MySQL Query Cache WhiteSpace and comments

By: roselynn

roselynn — Wed, 21 Oct 2009 16:01:02 +0000

is there a way to include whitespace / space characters in a query?

eg: ‘%to% go’

will return: ‘to stay or go’
but not: ‘tobago’

By: peter

peter — Fri, 05 Jun 2009 14:50:53 +0000

Good catch.

Hm…. Your results seems to show the comment upfront is not stripped – just does not block caching.
All second query run queries but timestamp query were instant. Timestamp query was also the same query just with different leading comment and it was not cached.

I’d consider it a bug.

By: Devis Lucato

Devis Lucato — Fri, 05 Jun 2009 10:42:19 +0000

I’ve run a very simple test using with a PHP script and a table with ~400k rows.
After restarting Mysql, to delete the cache, this is the result.

Mysql : 5.0.51a-24 (Debian)
PHP : 5.2.6-1+lenny2 with Suhosin-Patch 0.9.6.2 (cli) (built: Jan 26 2009 22:41:04)

FIRST RUN

/* comment 1 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.823220014572

/* comment 2 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.852579116821 seconds

/* comment 3 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.84706401825 seconds

/* comment 4 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.83235502243 seconds

/* comment 5 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.859950065613 seconds

/* timestamp 1244198091 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.470226049423 seconds

SECOND RUN

/* comment 1 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.00132608413696

/* comment 2 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.00090503692627 seconds

/* comment 3 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.000951051712036 seconds

/* comment 4 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.000913858413696 seconds

/* comment 5 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.000999927520752 seconds

/* timestamp 1244198149 */ SELECT count(*) FROM `mytable` WHERE `myfield` LIKE “%c%”
0.39192199707 seconds

I’d say that injecting dynamic comments clearly affects caching negatively, more memory consumption and slower results.

By: SQL Formatting » Bogatas.Org

SQL Formatting » Bogatas.Org — Thu, 12 Mar 2009 08:29:34 +0000

[...] Whitespace is ignored by MySQL’s parser, but not by MySQL’s query cache on versions <5 , thanks to foobar for pointing that out. So if you want query cache to pay off.. Pick one format [...]

By: peter

peter — Tue, 25 Mar 2008 18:44:49 +0000

Thanks indeed there is an option to skip comments now and you should run mysql -c if you want do to this kind of resting with command line client

By: jenni s

jenni s — Tue, 25 Mar 2008 16:44:32 +0000

it’s the same: the perl DBI doesn’t strip out whitespace or comments, but it will strip out whitespace at the beginning of comments.

By: jenni s

jenni s — Tue, 25 Mar 2008 16:37:53 +0000

My understanding is that the mysql command line client now has an *option* to pass comments to the server, but it’s disabled by default:
http://bugs.mysql.com/bug.php?id=26215

I’m running the 5.0.56 client and server, and from the command line, my comment was stripped out (as seen by the query recorded in the slow query log). However, when I ran this php script:

I *did* see the comment in the slow log:
# Time: 080325 9:31:43
# User@Host: dbadmin[dbadmin] @ localhost []
# Query_time: 2 Lock_time: 0 Rows_sent: 230400 Rows_examined: 480
select /* my comment */ * from test.WEBSITE a, test.WEBSITE b;

By: peter

peter — Fri, 21 Mar 2008 03:55:51 +0000

As far as I know PHP does not do any conversion to the query passed, so whitespace remains whitespace. Though you’re welcome to check it with DBI and let me know if it is different

By: Justin Swanhart

Justin Swanhart — Thu, 20 Mar 2008 23:10:03 +0000

Is PHP stripping off the whitespace from the front of the query? I’m curious if your same test run under Perl::DBI would behave consistently with the PHP test.