Tag: mysql

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hotel booking / wp_mphb_sync_logs

Just a random post about a WordPress plugin (hotel booking).

For the site in question, I have a script running which alerts me to any long running (>600s) MySQL queries (or causes of deadlock) and attempts to kill them. When it does this it emails me….

So, the site/MySQL was trying to run queries like this :

DELETE FROM wp_mphb_sync_logs WHERE queue_id IN (SELECT queue_id FROM wp_mphb_sync_queue WHERE queue_name < '1639573980');

which I did an explain, it showed there were > 5 million rows to examine (none of which are actually deleted by the query, so I assume the 5 million rows are all for now invalid queue_id entries).

Adding an index on the wp_mphb_sync_logs.queue_id field didn’t really help speed up the delete … and googling around and checking the plugin’s source code, led me to think it was safe (enough) to do a ‘TRUNCATE wp_mphb_sync_logs‘.

Now that’s done the table has remained empty 12 hours later; so I think everything’s fine.

This post is mostly because the plugin’s forum requires a paid membership to contribute; and I’m not paying $400 just to post “doing this worked for me”.

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Bash / MySQL queries…

I “woke up” and realised I often do this wrong …. (too many connections/individual queries on MySQL) when the query is returning simple values (single word like things) for each field.

As a means of example :

So a before query :

MYSQL="mysql -NB my_database"
IDENTIFIER=some_value

# Obviously $VAR1/$VAR2 can contain spaces etc in this example
VAR1=$(echo "SELECT field FROM table WHERE some_key = '${IDENTIFIER}'" | $MYSQL)
VAR2=$(echo "SELECT field FROM table WHERE some_key = '${IDENTIFIER}'" | $MYSQL)
....

And perhaps a nicer way might look more like :

#!/bin/bash
# Chuck output from a query into an array (retrieve many fields at once)
# coalesce( ... ) copes with a field that may be null.

BITS=( $(echo "SELECT coalesce(field1,200) as field1, field2, field3 FROM some_table WHERE some_key = '${IDENTIFIER}'" | $MYSQL ) )

# and then check we had enough values back
if [ ${#BITS[@] -lt 3 ]; then
    echo "handle this error..."
fi
# This will not work if field1/field2/field3 contain spaces.
VAR1=${BITS[0]}
VAR2=${BITS[1]}
VAR3=${BITS[2]}

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Cleaning up a lot of WordPress _transients

WordPress seems to like hiding a load of ‘transient’ (cacheable) stuff in it’s wp_options table. Unfortunately for one site, it seems it didn’t bother to clean up the transient stuff, leaving behind about 750,000 records… which made a WP version upgrade painful, as MySQL locks the wp_option table which causes all other page loads to get stuck waiting (and the site to stop working).

Fix / hacky solution after the ‘more’…

Continue reading “Cleaning up a lot of WordPress _transients”

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MySQL update/write query analysis (query profiling)

Do you have a slow MySQL update/insert/delete query?

Obviously, for ‘SELECT’ queries you can prepend the query with “EXPLAIN ” – however that doesn’t work for the other query types (UPDATE/INSERT/DELETE).

So, one solution which may explain why the query is slow is to turn on MySQL’s profiling functionality, like in the following example :
Continue reading “MySQL update/write query analysis (query profiling)”

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Late to the performance party

Everyone else probably already knows this, but $project is/was doing two queries on the MySQL database every time the end user typed in something to search on

  1. to get the data between a set range (SELECT x,y….. LIMIT n, OFFSET m or whatever) and
  2. another to get the total count of records (SELECT count(field) ….).

This is all very good, until there is sufficiently different logic in each query that when I deliberately set the offset in query #1 to 0 and limit very high and find that the of rows returned by both doesn’t match (this leads to broken paging for example)

Then I thought – surely everyone else doesn’t do a count query and then repeat it for the range of data they want back – there must be a better way… mustn’t there?

At which point I found:
http://forge.mysql.com/wiki/Top10SQLPerformanceTips
 and
http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_found-rows

See also the comment at the bottom of http://php.net/manual/en/pdostatement.rowcount.php which gives a good enough example (Search for SQL_CALC_FOUND_ROWS)

A few modifications later, run unit tests… they all pass…. all good.

I also found some interesting code like :

$total = sizeof($blah);
if($total == 0) { … }
elseif ($total != 0) { …. }
elseif ($something) { // WTF? }
else { // WTF? }

(The WTF comment were added by me… and I did check that I wasn’t just stupidly tired and not understanding what was going on).

The joys of software maintenance.