On 2013-07-12 15:53, Lukas Eder wrote:
OK, I'm going to be a bit sarcastic in this mail. I hope you're not
offended.
Not at all.
Now, if I specify OFFSET 600000 and H2 will evaluate the projection
for all the 599999 records that are not of interest, then OFFSET might
be a bit slow, right? Consider this code from Select.queryFlat().
600000x the following piece of code
You are correct, that it is how we currently perform that query.
That's 600000 arrays put into the result arraylist, each with the
length of the number of columns in the queried table. I don't know if
this accounts for 10% or 90% of the OP's reported 20 seconds. But
600000 arrays with 17 columns (from the OP's table) is a lot of wasted
memory when skipping to OFFSET 600000.
You say waste, I say nice simple code.
Now, I'm not saying you're wrong, and maybe we can improve that.
But I'd like to see a real-world case first, and I'd also like to see
some profiling that points to that code as a problem.
Because at the moment our performance is pretty darn good, and a large
chunk of that is precisely because we don't go chasing benchmarks and
fixing problems that are only theoretical.
And this happens for non-silly queries, too. In fact, this happens for
every OFFSET clause unless I made a mistake? OFFSET 1000 is a more
real-world use-case, agreed. And as a side-effect of this issue, there
was also the wrong (or unexpected) FOR UPDATE behaviour.
Yeah, the side effect is that we lock a few more rows than we should.
But that's not a correctness issue, it's just a performance issue.
And this is my last word on this topic for today, further queries will
be > /dev/null
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