What does the COUNT function do and why do we need it? The answer is pretty simple: it counts records. The usage of COUNT with HAVING with an example.An example of using COUNT with GROUP BY.Using COUNT with the WHERE clause example.This article will explore one of the most frequently used functions in MySQL – the COUNT() function. No matter which tool you use, practical methods do exist, and they are well-known to database professionals. There are many convenient IDEs like dbForge Studio for MySQL with rich functionality letting you do the tasks automatically. It would consume all your time and raise plenty of risks. Does it mean you have to do them all manually? Of course, not. My reasoning is that I can not see an easy way of maintaining multiple different pre-computed counts without introducing any new global locking or overhead – which is a big no-no in being able to scale on multiple cores/cpus.įinally, of the three solutions, this is the only 100% accurate method to be able to tell the exact number of rows in the table.If your occupation has anything to do with databases, you face all kinds of routine tasks on data manipulation every day. This solution will not scale well as the number of rows in the table grows and while storage-engine development is outside of my expertise, I suspect it is unlikely that this will be improved in any future MySQL versions. The actual count(1) will depend on the time your transaction started, and its isolation level. In a practical sense what this feature means, is that at any one point in time there will be multiple versions of a row. InnoDB supports MVCC which is an important feature to allow concurrent access to rows without having to need readers set locks blocking other users from writing. It’s important to explain why that is, since this behavior differs from MyISAM. This requires an index scan of the primary key. The example given: mysql> SELECT COUNT(1) FROM line_items Pro tip: It is quite possible it may look much slower as soon as the table does not fit in memory, since 8 random pages could mean > 8 random IOs. So based on it being a fixed number of pages to examine, it is also going to scale reasonably with table growth. The option is now called innodb_stats_persistent_sample_pages – a reference to the new persistent statistics feature! By default this sampling is just 8 pages in MySQL 5.5 (configurable via innodb_stats_sample_pages), but is greatly improved in MySQL 5.6 – with 20 pages sampled by default. InnoDB samples a number of random pages in the table, and then estimates the total rows for the whole table. So the question is really about how efficient a count already provided is, and where does it come from. This method takes a number that InnoDB provides, which in this case is accurate to within 4% of the actual number of rows (estimating slightly over in this case). The example given: mysql> SELECT TABLE_ROWS FROM INFORMATION_SCHEMA.TABLES So as you can see the number it returns is just over 10% higher than the actual number of rows (9441296). For example a page file is only 15/16ths in InnoDB. I have actually never thought of using this method! I don’t think it’s accurate though, since data length has deleted space + additional preallocated or overhead space. The example given: mysql> select DATA_LENGTH/AVG_ROW_LENGTH from INFORMATION_SCHEMA.TABLES He had three examples of how to estimate the number of rows in a table and he wanted to know the difference between them. He was working on a method to ship application metrics to statsd on a 1 minute interval. jason on MySQL 5.6.24 Community Release NotesĪ friend wrote to me recently with a question.
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