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Re: Query Performance SQL Server vs. Postgresql

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Query Performance SQL Server vs. Postgresql
Date: 2010-11-17 20:47:31
Message-ID: 4CE43F63.2030806@fuzzy.cz (view raw or flat)
Thread:
Lists: pgsql-performance
Dne 17.11.2010 05:47, Pavel Stehule napsal(a):
> 2010/11/17 Humair Mohammed <humairm(at)hotmail(dot)com>:
>>
>> There are no indexes on the tables either in SQL Server or Postgresql - I am
>> comparing apples to apples here. I ran ANALYZE on the postgresql tables,

Actually no, you're not comparing apples to apples. You've provided so
little information that you may be comparing apples to cucumbers or
maybe some strange animals.

1) info about the install

What OS is this running on? I guess it's Windows in both cases, right?

How nuch memory is there? What is the size of shared_buffers? The
default PostgreSQL settings is very very very limited, you have to bump
it to a much larger value.

What are the other inportant settings (e.g. the work_mem)?

2) info about the dataset

How large are the tables? I don't mean number of rows, I mean number of
blocks / occupied disk space. Run this query

SELECT relname, relpages, reltuples, pg_size_pretty(pg_table_size(oid))
FROM pg_class WHERE relname IN ('table1', 'table2');

3) info about the plan

Please, provide EXPLAIN ANALYZE output, maybe with info about buffers,
e.g. something like

EXPLAIN (ANALYZE ON, BUFFERS ON) SELECT ...

4) no indexes ?

Why have you decided not to use any indexes? If you want a decent
performance, you will have to use indexes. Obviously there is some
overhead associated with them, but it's premature optimization unless
you prove the opposite.

BTW I'm not a MSSQL expert, but it seems like it's building a bitmap
index on the fly, to synchronize parallelized query - PostgreSQL does
not support that.

regards
Tomas

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