Re: Planner should use index on a LIKE 'foo%' query

Am 28.06.2008 um 21:19 schrieb Steinar H. Gunderson:

> On Sat, Jun 28, 2008 at 06:24:42PM +0200, Moritz Onken wrote:
>> SELECT distinct url from item where url like 'http://www.micro%'
>> limit
>> 10;
>
> Here, the planner knows the pattern beforehand, and can see that
> it's a
> simple prefix.
>> select *
>> from result
>> where exists
>> (select * from item where item.url LIKE result.url || '%' limit 1)
>> limit 10;
>
> Here it cannot (what if result.url was '%foo%'?).

That's right. Thanks for that hint. Is there a Postgres function which
returns a constant (possibly an escape function)?
>
>
> Try using something like (item.url >= result.url && item.url <=
> result.url ||
> 'z'), substituting an appropriately high character for 'z'.
>
>> The only explaination is that I don't use a constant when comparing
>> the
>> values. But actually it is a constant...

I created a new column in "item" where I store the shortened url which
makes "=" comparisons possible.

the result table has 20.000.000 records and the item table 5.000.000.
The query

select count(1) from result where url in (select shorturl from item
where shorturl = result.url);

will take about 8 hours (still running, just guessing). Is this
reasonable on a system with 1 GB of RAM and a AMD Athlon 64 3200+
processor? (1 SATA HDD)

regards,

moritz