Le jeudi 21 juin 2012 04:45:41, Craig Ringer a écrit : > 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. There was a recent thread on -hackers about index with UNIQUEness of some columns only. The objective was near the one you describe here. So you're not alone looking after that. -- Cédric Villemain +33 (0)6 20 30 22 52 http://2ndQuadrant.fr/ PostgreSQL: Support 24x7 - Développement, Expertise et Formation
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