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
