On 17 July 2012 16:54, David E. Wheeler <da...@justatheory.com> wrote:
> On Jul 17, 2012, at 5:32 PM, Simon Riggs wrote:
>>  CREATE INDEX ON foo (a, b, c, d);
>> allows
>>  SELECT c, d FROM foo WHERE a = ? AND b = ?
>> to use an index only scan.
>> The phrase "unindexed" seems misleading since the data is clearly in
>> the index from the description on the URL you gave. And since the
>> index is non-unique, I don't see any gap between Postgres and
>> SQLliite4.
> Yeah, but that index is unnecessarily big if one will never use c or d in the 
> search. The nice thing about covering indexes as described for SQLite 4 and 
> implemented in MSSQL is that you can specify additional columns that just 
> come along for the ride, but are not part of the indexed data:
>     CREATE INDEX cover1 ON table1(a,b) COVERING(c,d);
> Yes, you can do that by also indexing c and d as of 9.2, but it might be nice 
> to be able to include them in the index as additional row data without 
> actually indexing them.

Can you explain what you mean by "without actually indexing them"?
ISTM that it is a non-feature if the index is already non-unique, and
the difference is simply down to the amount of snake oil applied to
the descriptive text on the release notes.

 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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