Re: Sorting performance vs. MySQL

Op 22 feb 2010, om 19:30 heeft Richard Broersma het volgende geschreven:

> On Mon, Feb 22, 2010 at 10:17 AM, Frank Heikens
> <frankheikens(at)mac(dot)com> wrote:
>
>> There is no index on the column transactionid in your PostgreSQL-
>> table, as
>> there is in your MySQL-table. This explains the difference.
>>
>> CREATE INDEX i_transactionid ON public.metarelcloud_transactionlog
>> (transactionid);
>
> Does an index help a sort operation in PostgreSQL?

Yes it does, see the manual: http://www.postgresql.org/docs/8.4/interactive/indexes-ordering.html

Example without index:
"Sort (cost=804.39..829.39 rows=10000 width=4) (actual
time=16.006..17.171 rows=10000 loops=1)"
" Sort Key: bar"
" Sort Method: quicksort Memory: 491kB"
" -> Seq Scan on bla (cost=0.00..140.00 rows=10000 width=4) (actual
time=0.015..2.236 rows=10000 loops=1)"
"Total runtime: 18.098 ms"

Same query with index (btree):
"Index Scan Backward using i_bar on bla (cost=0.00..406.25 rows=10000
width=4) (actual time=0.093..4.408 rows=10000 loops=1)"
"Total runtime: 5.381 ms"

>
>
> --
> Regards,
> Richard Broersma Jr.
>
> Visit the Los Angeles PostgreSQL Users Group (LAPUG)
> http://pugs.postgresql.org/lapug

Regards,
Frank Heikens