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I have 1 million rows in MySql table “temp” and wish to multiply column “t” (int unsigned, indexed) by 1000.
mysql> update temp set t=1000*t;
This process takes 25 seconds. The same statement on not-indexed column takes 10 seconds;
Any ideas how to make this process faster? I have to apply this on over 1e5 tables.
Indexing has nothing to do with the problem here. Think about what you’re doing – you’re mutating all the rows in your table, so no matter how you select them and if you have an index on t or not, you’re still scanning the whole table.
The UPDATE operation == IO is what your bottleneck is. Get faster disks.
If you’re using InnoDB, my only advice would be to see if tweaking innodb_flush_log_at_trx_commit and setting it to 2 helps your performance but I doubt it as it’s just 1 query. Disabling keys and re-enabling them after UPDATE won’t work in InnoDB.