Today I wanted to take a look at improving the performance of tables that cause performance problems based largely on their size. Some of this advice also applies to databases that are large in-aggregate over many tables, but I always find the individually large table a special-case that is problematic.
What you will normally find is that the speed that the table can be modified will trend down as the size increases. Here is what I am going to call the typical B+Tree index performance over time:
So we should expect degradation of performance due to the structure of the index, but there are actually some ways that we can try and stretch out the curve, and not degrade as quickly.
Ten potential ways to reduce large table impact:
Make sure to use InnoDB instead of MyISAM. MyISAM can be faster at inserts to the end of a table, but it has both table locking (limiting updates and deletes) and uses a single lock to protect the key buffer when loading data to/from disk, resulting in contention. It also does not have the change buffering feature described just below.
InnoDB has change buffering (previously called the insert buffer), which is a feature to delay building secondary indexes that are not unique, and merge writes. It's further described by Facebook here. It's not shown in the graph above, but it can boost insert performance by quite a lot, and it's enabled by default. It was greatly improved in MySQL 5.5, so it is time to upgrade if you haven't.
Partitioning may reduce the size of indexes, effectively reducing the table
into many smaller tables. It also reduces internal index->lock contention, something that has been greatly improved in the MySQL 5.7.2 DMR.
Use innodb page compression. For some workloads (particularly those with lots of char/varchar/text data types) compression will allow the data to be more compact, stretching out that performance curve for longer. It may also allow you to more easily justify SSDs which are typically smaller in capacity. InnoDB page compression was improved a lot in MySQL 5.6, courtesy of Facebook providing a series of patches.
Sort and bulk load data into tables. Inserting in order will result in fewer page splits (which will perform worse on tables not in memory), and the bulk loading is not specifically related to the table size, but it will help reduce redo log pressure.
Remove any unnecessary indexes on the table, paying particular attention to UNIQUE indexes as these disable change buffering. Don't use a UNIQUE index if you have no reason for that constraint; prefer a regular INDEX.
Related to the points 5 & 6, the type of primary key also matters. It is much better to use either an
BIGINTdatatype than say a
GUID, which will have a curve that degrades much faster. Having no PRIMARY KEY will also affect performance negatively.
If bulk loading a fresh table, delay creating any indexes besides the PRIMARY KEY. If you create them once all data is loaded, then InnoDB is able to apply a pre-sort and bulk load process which is both faster and results in typically more compact indexes. This optimization became true in MySQL 5.5.
More memory can actually help here too. I frequently see people under spec memory on new database servers compared to what it actually costs these days. Simple advice: If
SHOW ENGINE INNODB STATUSshows any reads/s under
BUFFER POOL AND MEMORYand the number of Free buffers (also under
BUFFER POOL AND MEMORY) is zero, you could benefit from more (assuming you have sized
innodb_buffer_pool_sizecorrectly on your server. See here.)
As well as memory, SSDs can help too. Much of the performance drop shown on the curve can be attributed to additional IO which is created as the table gets bigger. While a hard drive can do 200 operations per second (IOPS), a typical SSD will do 20K+