Re: [HACKERS] Surjective functional indexes

On 23.03.2018 18:45, Alvaro Herrera wrote:
> Konstantin Knizhnik wrote:
>
>> rd_projidx is not a list, it is Bitmapset. It is just one of many bitmap
>> sets in RelationData:
> Yes, but the other bitmapsets are of AttrNumber of the involved column.
> They new one is of list_nth() counters for items in the index list.
> That seems weird and it scares me -- do we use that coding pattern
> elsewhere?  Maybe use an array of OIDs instead?
>
Using list or array instead of bitmap requires much more space...
And bitmaps are much more efficient for many operations: check for 
element presence, combine, intersect,...
Check in bitmap has O(0) complexity so iterating through list of all 
indexes or attributes with bitmap check doesn't add any essential overhead.

Sorry, I do not understand this: "They new one is of list_nth() counters 
for items in the index list"

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company