Re: GSoC proposal. Index-only scans for GIST

2. gistget algorithm (several parts omitted):

Check the key
gistindex_keytest() – does this index tuple satisfy the scan key(s)?

Scan all items on the GiST index page and insert them into the queue (or directly to output areas)

plain scan

Heap tuple TIDs are returned into so->pageData[]

ordered scan

Heap tuple TIDs are pushed into individual search queue items

If the fetch() is specified by the developer, then using it, algorithm can retrieve the data directly to output areas at this stage, without reference to the heap.

3. Create function gistcanreturn to check whether fetch() is specified by user.

Amcanreturn – Function to check whether index supports index-only scans, or zero if none

There is the question of support index-only scans for multicolumn indexes. Probably it would require extend the interface to add separate columns checking.

To solve this question I need the community’s help.

4. Add details for index only scans into gistcostestimate function