On 10/18/2016 11:44 AM, Francisco Olarte wrote:
On Tue, Oct 18, 2016 at 5:06 PM, Andy Colson <andy@xxxxxxxxxxxxxxx> wrote:
I wanted to report an awesome performance boost using tablesample.
In my stored function I was getting a random row using:
select one into x from ones order by random() limit 1;
When the table was smaller it worked fine, but the performance has slowly
gotten worse. This morning I was getting around 8 transactions a second.
Which is not a surprise, as it has to at least read all the rows and
generate a random() for each one and keep track of the minimum.
I just replaced it with:
select one into x from ones tablesample bernoulli(1) limit 1;
This should be faster, but to me it seems it does a different thing.
This seems to select each row of the table with probability 1% and
return the first selected, i.e., something similar to
select one into x from ones where random()>0.01 limit 1.
Which has the ( diminishing with table size ) risk of selecting zero
rows and is going to select one of the first 100 or so rows with high
probability, unless I'm missing something.
I say this because docs state ir returns a 'randomly chosen', sample,
not a 'randomly ORDERED' one, and the straightforward implementation
of sampling returns rows in the primitive scan order. I supose it
could be easily tested by selecting bernouilli(100), but have not
server access now to verify it.
With a big table it seems:
select one into x from ones where random()>0.01 order by random() limit 1
or
select one into x from ones tablesample bernoulli(1) order by random() limit 1;
Is more similar to what you originally did ( and the run time should
possibly be something in between ).
I would recomend you to execute the function and verify it does what
you want ( as you say it's fast, I would try selecting a several
thousands and eyeballing the result, if it does what I fear the
grouping should be obvious ).
Maybe you do not mind it, in which case it's ok, but a one minute run
should let you know wahat you are exactly doing.
Francisco Olarte.
Ah, yes, you're right, there is a bit of a difference there.
Speed wise:
1) select one from ones order by random() limit 1;
> about 360ms
2) select one from ones tablesample bernoulli(1) limit 1 ;
> about 4ms
3) select one from ones tablesample bernoulli(1) order by random() limit 1;
> about 80ms
Using the third option in batch, I'm getting about 15 transactions a second.
Oddly:
select one from ones tablesample bernoulli(0.25) order by random()
takes almost 80ms also.
bernoulli(0.25) returns 3k rows
bernoulli(1) returns 14k rows
Thanks,
-Andy
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