On 18 October 2016 at 22:04, Claudio Freire <klaussfre...@gmail.com> wrote:
> On Tue, Oct 18, 2016 at 3:28 PM, Alvaro Herrera
> <alvhe...@2ndquadrant.com> wrote:
>> I propose we introduce the concept of "indirect indexes".  I have a toy
>> implementation and before I go further with it, I'd like this assembly's
>> input on the general direction.
>> Indirect indexes are similar to regular indexes, except that instead of
>> carrying a heap TID as payload, they carry the value of the table's
>> primary key.  Because this is laid out on top of existing index support
>> code, values indexed by the PK can only be six bytes long (the length of
>> ItemPointerData); in other words, 281,474,976,710,656 rows are
>> supported, which should be sufficient for most use cases.[1]
> You don't need that limitation (and vacuum will be simpler) if you add
> the PK as another key, akin to:
> CREATE INDIRECT INDEX idx ON tab (a, b, c);
> turns into
> CREATE INDEX idx ON tab (a, b, c, pk);
> And is queried appropriately (using an index-only scan, extracting the
> PK from the index tuple, and then querying the PK index to get the
> tids).
> In fact, I believe that can work with all index ams supporting index-only 
> scans.

But if you did that, an UPDATE of a b or c would cause a non-HOT
update, so would defeat the purpose of indirect indexes.

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

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