Re: how fast index works?

Thanks!

De: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] En nombre de Craig James
Enviado el: martes, 06 de septiembre de 2011 03:18 p.m.
Para: pgsql-performance(at)postgresql(dot)org
Asunto: Re: [PERFORM] how fast index works?

On 9/6/11 11:31 AM, Anibal David Acosta wrote:

Hi everyone,

My question is, if I have a table with 500,000 rows, and a SELECT of one row
is returned in 10 milliseconds, if the table has 6,000,000 of rows and
everything is OK (statistics, vacuum etc)

can i suppose that elapsed time will be near to 10?

Theoretically the index is a B-tree with log(N) performance, so a larger
table could be slower. But in a real database, the entire subtree might
fall together in one spot on the disk, so retrieving a record from a 500,000
record database could take the same time as a 6,000,000 record database.

On the other hand, if you do a lot of updates and don't have your autovacuum
parameters set right, a 500,000 record index might get quite bloated and
slow as it digs through several disk blocks to find one record.

There is no simple answer to your question. In a well-maintained database,
6,000,000 records are not a problem.

Craig