Re: Postgres 9.0 has a bias against indexes

On Thu, Jan 27, 2011 at 2:12 PM, Igor Neyman <ineyman(at)perceptron(dot)com> wrote:
>
>
>> -----Original Message-----
>> From: Scott Marlowe [mailto:scott(dot)marlowe(at)gmail(dot)com]
>> Sent: Thursday, January 27, 2011 3:59 PM
>> To: Mladen Gogala
>> Cc: Igor Neyman; Tom Lane; David Wilson; Kenneth Marshall;
>> pgsql-performance(at)postgresql(dot)org
>> Subject: Re: [PERFORM] Postgres 9.0 has a bias against indexes
>>
>> On Thu, Jan 27, 2011 at 1:44 PM, Mladen Gogala
>> <mladen(dot)gogala(at)vmsinfo(dot)com> wrote:
>> > On 1/27/2011 3:37 PM, Scott Marlowe wrote:
>> >>
>> >> On Thu, Jan 27, 2011 at 1:31 PM, Mladen Gogala
>> >> <mladen(dot)gogala(at)vmsinfo(dot)com>  wrote:
>> >>>
>> >>> There is INDEX UNIQUE SCAN PK_EMP.  Oracle will use an index.
>> >>
>> >> That's because Oracle has covering indexes.
>> >>
>> > I am not sure what you mean by "covering indexes" but I
>> hope that for
>> > the larger table I have in mind,  indexes will be used.
>> For a small
>> > table like
>>
>> In Oracle you can hit JUST the index to get the data you need
>> (and maybe rollback logs, which are generally pretty small)
>>
>> In Pgsql, once you hit the index you must then hit the actual
>> data store to get the right version of your tuple.  So, index
>> access in pg is more expensive than in Oracle.  However,
>> updates are cheaper.
>> Always a trade off
>>
>>
>
> Scott,
> What you describe here isn't about "covering indexes" - it's about different ways implementing MVCC in Oracle and PG.

It is about covering indexes AND it's about the difference in how MVCC
is implemented in both databases.