Re: Index-only scans for multicolumn GIST

On Tue, Jul 22, 2014 at 2:55 AM, Heikki Linnakangas
<hlinnakangas(at)vmware(dot)com> wrote:
> On 07/21/2014 10:47 PM, Anastasia Lubennikova wrote:
>>
>> Hi, hackers!
>> There are new results of my work on GSoC project "Index-only scans for
>> GIST".
>> Previous post is here:
>>
>> http://postgresql.1045698.n5.nabble.com/Index-only-scans-for-GIST-td5804892.html
>>
>> Repository is
>> https://github.com/lubennikovaav/postgres/tree/indexonlygist2
>> Patch is in attachments.
>> It includes indexonlyscan for multicolumn GiST. It works correctly -
>> results are the same with another scan plans.
>> Fetch() method is realized for box and circle opclasses
>> Improvement for circle opclass is not such distinct as for box opclass,
>> because of recheck.
>
>
> For a circle, the GiST index stores a bounding box of the circle. The new
> fetch function reverses that, calculating the radius and center of the
> circle from the bounding box.
>
> Those conversions lose some precision due to rounding. Are we okay with
> that? Floating point math is always subject to rounding errors, but there's
> a good argument to be made that it's not acceptable to get a different value
> back when the user didn't explicitly invoke any math functions.
>
> As an example:
>
> create table circle_tbl (c circle);
> create index circle_tbl_idx on circle_tbl using gist (c);
> insert into circle_tbl values ('1.23456789012345,1.23456789012345,1e300');
>
> postgres=# set enable_seqscan=off; set enable_bitmapscan=off; set
> enable_indexonlyscan=on;
> SET
> SET
> SET
> postgres=# select * from circle_tbl ;
> c
> ----------------
> <(0,0),1e+300>
> (1 row)
>
> postgres=# set enable_indexonlyscan=off;
> SET
> postgres=# select * from circle_tbl ;
> c
> ----------------------------------------------
> <(1.23456789012345,1.23456789012345),1e+300>
> (1 row)

I really don't think it's ever OK for a query to produce different
answers depending on which plan is chosen.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company