On Thu, Mar 2, 2017 at 1:19 PM, Sven R. Kunze <srkunze@mail.de> wrote:
On 01.03.2017 18:04, Jeff Janes wrote:
On Wed, Mar 1, 2017 at 6:02 AM, Sven R. Kunze <srkunze@mail.de> wrote:
On 28.02.2017 17:49, Jeff Janes wrote:
Oh. In my hands, it works very well. I get 70 seconds to do the {age: 20} query from pure cold caches, versus 1.4 seconds from cold caches which was followed by pg_prewarm('docs','prefetch').
How much RAM do you have? Maybe you don't have enough to hold the table in RAM. What kind of IO system? And what OS?
4GB is not much RAM to be trying to pre-warm this amount of data into. Towards the end of the pg_prewarm, it is probably evicting data read in by the earlier part of it.
What is shared_buffers?
942MB.
But I see where you are coming from. How come that these queries need a Recheck Cond? I gather that this would require reading not only the index data but also the table itself which could be huge, right?
Bitmaps can overflow and drop the row-level information, tracking only the blocks which need to be inspected. So it has to have a recheck in case that happens (although in your case it is not actually overflowing--but it still needs to be prepared for that). Also, I think that jsonb_path_ops indexes the hashes of the paths, so it can deliver false positives which need to be rechecked. And you are selecting `id`, which is not in the index so it would have to consult the table anyway to retrieve that. Even if it could get all the data from the index itself, I don't think GIN indexes support that feature.
