One of the biggest and undead myths in SQL is that COUNT(*) is faster than COUNT(1). Or was it that COUNT(1) is faster than COUNT(*)? Impossible to remember, because there's really no reason at all why one should be faster than the other. But is the myth justified? Let's measure! How does COUNT(...) work? But … Continue reading What’s Faster? COUNT(*) or COUNT(1)? →
The SQL language and its depths... Some of you readers might be aware of MySQL's capability of counting distinct records through the COUNT() aggregate function. The MySQL documentation reads: COUNT(DISTINCT expr,[expr...]) Returns a count of the number of rows with different non-NULL expr values. In other words, you can count distinct first and last names very easily: SELECT … Continue reading Counting Distinct Records in SQL →
There is not magic solution to count table contents especially if you have a client that has a table with 40+ million records and you need to do the filtering using the 'OR' SQL statement.
Original query was the following:
SELECT COUNT(*) from table1 WHERE field1 IN ('val1','val2') OR field2 IN ('val3','val4');
First benchmark before doing optimization showed me results after 4 minutes.
This blag was originally posted at http://cafuego.net/2010/05/26/fast-paging-real-world
Some time ago I attended the “Optimisation by Design” course from Open Query¹. In it, Arjen teaches how writing better queries and schemas can make your database access much faster (and more reliable). One such way of optimising things is by adding appropriate query hints or flags. These hints are magic strings that control how a server executes a query or how it returns results.
An example of such a hint is SQL_CALC_FOUND_ROWS. You use it in a select query with a LIMIT clause. It instructs the server to select a limited numbers of rows, but also to calculate the total number of rows that would have been returned without the limit clause in place. That total number of rows is stored in a session variable, which can be retrieved …[Read more]
So this is about a SELECT COUNT(*) FROM tblname without a WHERE clause. MyISAM has an optimisation for that since it maintains a rowcount for each table. InnoDB and PBXT can’t do that (at least not easily) because of their multi-versioned nature… different transactions may see a different number of rows for the table table!
So, it’s kinda known but nevertheless often ignored that this operation on InnoDB is costly in terms of time; what InnoDB has to do to figure out the exact number of rows is scan the primary key and just tally. Of course it’s faster if it doesn’t have to read a lot of the blocks from disk (i.e. smaller dataset or a large enough buffer pool).
I was curious about PBXT’s performance on this, and behold it appears to be quite a bit faster! For a table with 50 million rows, PBXT took about 20 minutes whereas the same table in InnoDB took 30 minutes. Interesting! …[Read more]
If your WordPress comment counts got messed up, whether because of a plugin (I'm talking about you, DISQUS) or you messed with your database manually and did something wrong (yup, that's what I just did), fear not – I have a solution for you.
But first, a little background.
Comment Counts In WordPress
Here's how comment counts work in WP:
- Posts live in a table called wp_posts and each has an ID.
- Comments reside in a table called wp_comments, each referring to an ID in wp_posts.
- However, to make queries faster, the comment count is also
cached in the wp_posts table, rather than getting calculated on
every page load.
If this count ever gets out of sync with the actual number of comments for some reason, WordPress, while still displaying all comments properly, will simply show the wrong count. …