----- Original Message -----
> From: "Hank" <heskin@stripped>
> (please read my ORIGINAL post with all this information).
Welcome on the Internet, where everyone will tell you everything you need to know, except
for what you want to know :-)
> I am trying to find a logical or reasonable explanation WHY this would be the
> case, despite the fact that the documentation states otherwise (see: Right here:
I believe you're misinterpreting that, as is the author from the blog you originally
What it says, is "If you are going to run many operations". You are updating many rows,
but you are only doing ONE operation: a single update statement.
While this explains why you're not seeing benefit, I have to admit that I'm at a loss,
too, as to why you are experiencing an actual slowdown - the update statement will lock
the tables, too, so it shouldn't really make any difference at all.
> But if seeing some SQL will make you happy, here is just one example:
> UPDATE dest d straight_join source s set d.seq=s.seq WHERE d.key=s.key;
See, this is why people ask to see your queries. You never mentioned you were doing a join
in the update :-)
I'll ignore the join condition in the where clause, as it makes little difference in this
case; but I do note that you're using a straight join. Is the optimizer really reading
the tables in the wrong order, or is it just something you picked up off a blog without
realising the impact?
> Source is indexed by key+seq (key is primary key, but seq is
> included as a covering index).
Good practice, that should prevent source from being read from disk, if your index is
fully in the cache. Do you have an index on dest.key, too? That might help performance as
well if it fits in memory, too, because you'll only need disk access for flushing writes,
> This query takes about 3.5 hours when I don't use LOCK TABLES, and over 4 hours when
> I do use LOCK TABLES.
Most peculiar. Is the difference in performance consistent in repeated executions?
> And before testing each run, I do restart the server so there is no
> query caching and I also use FLUSH TABLES between each test run.
That's good, as it will give you the worst-case scenario. However, since such an update is
going to wreck your index cache anyway, you may just as well preload the appropriate
indices into it beforehand, if the cache is sized big enough to hold them. That might
give a minor performance boost, too, as the server won't have to go to disk every so
often to fetch index blocks.
See http://dev.mysql.com/doc/refman/5.0/en/index-preloading.html for that.
> | key_buffer_size | 402653184 |
400MB of key buffer, that's not bad at all. Do a /show table status/ in your database, and
sum the index sizes. If your key buffer is larger than this (and why not scale it for
growth a bit?) all your indices will fit, which will save on disk access for index
lookups *and* for index-covered queries.
Bier met grenadyn
Is als mosterd by den wyn
Sy die't drinkt, is eene kwezel
Hy die't drinkt, is ras een ezel