| From: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
|---|---|
| To: | David Johnston <polobo(at)yahoo(dot)com> |
| Cc: | "'David E(dot) Wheeler'" <david(at)justatheory(dot)com>, 'Simon Riggs' <simon(at)2ndquadrant(dot)com>, 'Pg Hackers' <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Covering Indexes |
| Date: | 2012-07-17 17:01:34 |
| Message-ID: | 50059A6E.1030503@dunslane.net |
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| Lists: | pgsql-hackers |
On 07/17/2012 12:41 PM, David Johnston wrote:
>>
>> So the question that needs to be asked is what kind of performance increase
>> can be had during DML (insert/update) statements and whether those gains are
>> worth pursuing. Since these other engines appear to allow both cases you
>> should be able to get at least a partial idea of the performance gains
>> between "index (a,b,c,d)" and "index (a,b) covering (c,d)".
Tom's recent answer to me on this point (as I understood it) was that he
would expect performance to degrade, not improve, since the btree code
is known not to perform well when there are many non-unique values.
cheers
andrew
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