On Mon, Jul 13, 2015 at 3:54 PM, Ben Hoyt <benhoyt@gmail.com> wrote:
> Hi folks,
>
> I have a fairly simple three-table query (pasted below) with two LEFT JOINs
> and an OR in the WHERE clause that for some reason is doing sequential scans
> on all three tables (two of them large -- several million rows), even though
> I have indexes on the relevant "filename" columns.
>
> Note the two parts of the where clause -- a filter on the image2 table and a
> filter on the panoramas table. If I comment out either filter and just
> filter on i.filename by itself, or p.filename by itself, the query planner
> uses the relevant index and the query takes a few milliseconds. But when I
> have both clauses (as shown below) it falls back to sequential scanning all
> three tables for some reason, taking several seconds.
>
> What am I missing? There must be some reason PostgreSQL can't use the index
> in this case, but I can't see what it is. If I were PostgreSQL I'd be using
> the index on i.filename and p.filename to filter to a couple of rows first,
> then join, making it super-quick.
>
> In this test I'm running PostgreSQL 9.3.3 on Windows 64-bit, but the same
FYI, this won'f fix your issue, but upgrade your postgres to the
latest bugfix release, 9.3.9.
> My query and PostgreSQL version and the explain and a lot of other table
> data is pasted below.
>
> QUERY
> ----------
> select ai.position, i.filename as image_filename, p.filename as
> panorama_filename
> from album_items ai
> left join image2 i on i.imageid = ai.image_id
> left join panoramas p on p.id = ai.panorama_id
> where i.filename in ('pano360--v471.jpg', 'pano360-2--v474.jpg') or
> p.filename in ('pano360--v471', 'pano360-2--v474')
Try refactoring to:
select ai.position, i.filename as image_filename, p.filename as
panorama_filename
from album_items ai
left join image2 i on i.imageid = ai.image_id
left join panoramas p on p.id = ai.panorama_id
where i.filename in ('pano360--v471.jpg', 'pano360-2--v474.jpg')
union all select ai.position, i.filename as image_filename, p.filename
as panorama_filename
from album_items ai
left join image2 i on i.imageid = ai.image_id
left join panoramas p on p.id = ai.panorama_id
where p.filename in ('pano360--v471', 'pano360-2--v474')
...and see if that helps. Dealing with 'or' conditions is a general
weakness of the planner that has gotten better over time but in some
cases you have to boil it to 'union all'.
merlin