|From:||Craig Ringer <ringerc(at)ringerc(dot)id(dot)au>|
|Cc:||Eyal Wilde <eyal(at)impactsoft(dot)co(dot)il>, pgsql-performance(at)postgresql(dot)org|
|Subject:||Re: index-only scan is missing the INCLUDE feature|
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On 06/20/2012 11:32 PM, Shaun Thomas wrote:
> On 06/20/2012 09:11 AM, Craig Ringer wrote:
>> For those of us who don't know MS-SQL, can you give a quick
>> explanation of what the INCLUDE keyword in an index definition is
>> expected to do, or some documentation references?
> He's talking about what MS SQL Server commonly calls a "covering
> index." In these cases, you can specify columns to be included in the
> index, but not actually part of the calculated hash. This prevents a
> trip to the table data, so selects can be serviced entirely by an
> index scan.
Oh, OK, so it's a covering index with added fields that don't form part
of the searchable index structure to make the index a little less
expensive than a fully covering index on all the columns of interest.
Fair enough. Thanks for the explanation.
Eyal, you'll get a better response to questions about other DBMSs if you
explain what you need/want to do with the desired feature and what that
feature does in the other DBMS.
> PostgreSQL is about half way there by allowing index-only scans,
> though I've no idea if they intend on adding further functionality
> like this.
There's certainly lots of interest in adding more, but not that many
people with the expertise to be able to do it - and fewer still who're
paid to work on Pg so they have time to focus on it. Covering indexes
with Pg's MVCC model seem to be particularly challenging, too.
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