Why do MySQL InnoDB inserts / updates on large tables get very slow when there are a few indexes?

user_159070 0
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We have a series of tables that have grown organically to several million rows, in production doing an insert or update can take up to two seconds. However if I dump the table and recreate it from the dump queries are lightning fast.

We have rebuilt one of the tables by creating a copy rebuilding the indexes and then doing a rename switch and copying over any new rows, this worked because that table is only ever appended to. Doing this made the inserts and updates lightning quick.

My questions:

Why do inserts get slow over time? Why does recreating the table and doing an import fix this? Is there any way that I can rebuild indexes without locking a table for updates?

indexing mysql performance
5 Answers

It sounds like it's either

  • Index unbalancing over time
  • Disk fragmentation
  • Internal innodb datafile(s) fragmentation

You could try analyze table foo which doesn't take locks, just a few index dives and takes a few seconds.

If this doesn't fix it, you can use

mysql> SET PROFILING=1;
mysql> INSERT INTO foo ($testdata);
mysql> show profile for QUERY 1;

and you should see where most of the time is spent.

Apparently innodb performs better when inserts are done in PK order, is this your case?

user_2467620

Updating a table requires indices to be rebuilt. If you are doing bulk inserts, try to do them in one transaction (as the dump and restore does). If the table is write-biased I would think about dropping the indices anyway or let a background job do read-processing of the table (eg by copying it to an indexed one).

user_1245360
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