Re: huge discrepancy between EXPLAIN cost and actual time (but the table has just been ANALYZED)

On 5/8/23 07:29, Kent Tong wrote:
> Hi,
>
> I have a complex query involving over 15 joins and a CTE query and it
> takes over 17s to complete. The output of EXPLAIN ANALYZE includes
> (somewhere deep inside):
>
> Index Scan using document_pkey on document document0_
>  (cost=0.29..8.31 rows=1 width=3027) (actual time=16243.959..16243.961
> rows=1 loops=1)
>
>
> This shows an index scan with a very small cost but a very large actual
> time. The strange thing is, all the tables have just been analyzed with
> the ANALYZE command (it is not a foreign table). Furthermore, if I run a
> simple query using that index, both the cost and the actual time are small.
>
> Another snippet is:
>
>                                   -> CTE Scan on all_related_document
> p  (cost=1815513.32..3030511.77 rows=241785 width=16) (actual
> time=203.969..203.976 rows=0 loops=1)
>
>
> I think the cost-actual time discrepancy is fine as it is a recursive CTE
> so postgresql can't estimate the cost well. It is materialized and a full
> table scan is performed. However, the actual time is not that bad.  Also,
> the estimated rows and the actual rows are also vastly different, but I
> guess this is fine, isn't it?
>
> Any idea how I should check further?

ANALYZE just samples the table.  If data within the relevant indexed columns
aren't evenly distributed, then the statistics might not show the true data
distribution.

--
Born in Arizona, moved to Babylonia.