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.

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