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

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?

Many thanks in advance

--
Kent Tong
IT author and consultant, child education coach