> On 30 Dec 2015, at 17:02, Tom Lane <t...@sss.pgh.pa.us> wrote:
>
> Oleksii Kliukin <al...@hintbits.com> writes:
>> Bitmap Heap Scan on example (cost=744.44..757.64 rows=6 width=0) (actual
>> time=73.895..73.895 rows=0 loops=1)
>> Output: 1
>> Recheck Cond: (example.event_time = (now() - '5 mons'::interval))
>> Rows Removed by Index Recheck: 4030
>> Heap Blocks: lossy=128
>> Buffers: shared hit=629
>> -> Bitmap Index Scan on example_event_time_idx1 (cost=0.00..744.41
>> rows=6 width=0) (actual time=70.335..70.335 rows=1280 loops=1)
>> Index Cond: (example.event_time = (now() - '5 mons'::interval))
>> Buffers: shared hit=501
>
>> - how does it get 1280 rows from the BRIN index scan, given that BRIN only
>> stores pointers to the heap blocks, not individual rows. Does it calculate
>> the number of rows in the blocks returned?
>
> It evidently returned 128 block IDs to the heapscan logic. I have not
> looked at the code, but a reasonable bet is that it's just guessing that
> there are 10 rows per block.
You are right, this is at the end of bringetbitmap in brin.c
/*
* XXX We have an approximation of the number of *pages* that our scan
* returns, but we don't have a precise idea of the number of heap
tuples
* involved.
*/
PG_RETURN_INT64(totalpages * 10);
>
> That seems like an awfully low number, though; it equates to assuming
> that rows are 800 bytes wide on average. If we're going to use a fixed
> number, 100 rows per block would probably be more nearly the correct
> order of magnitude.
>
> Another idea would be to use the heap's row density as calculated
> by the last ANALYZE (ie, reltuples/relpages), with a fallback to 100
> if relpages=0. This'd only be convenient if the bitmap scan node has
> the parent heap rel open, which it might not.
+1
Kind regards,
--
Oleksii
