I played around with COST up to 999999 – still no improvement.
HOWEVER – I determined that more than 95% of the records in the database are
local addresses and the indexed function returns NULL.
This seems to create an issue with LEFT JOIN.
Increasing the COST of the function to 20000 and rewriting the query as follows
provides a significantly better result:
explain analyse
SELECT
GCountry.szISOAlpha2,
GCountry.szISOAlpha3,
GCountry.szISONum3
from
t.order torder
join G.Country GCountry ON G.GetXmlAddressCountryID(TOrder.szXmlAddress) =
GCountry.oID
UNION ALL
SELECT
'XX'::TEXT, -- can use NULL::TEXT with no difference
'XXX'::TEXT, -- can use NULL::TEXT with no difference
'999'::TEXT -- can use NULL::TEXT with no difference
from
t.order torder
WHERE G.GetXmlAddressCountryID(TOrder.szXmlAddress) IS NULL
Gives:
Append (cost=0.29..7661.82 rows=13578 width=11) (actual time=0.106..20.464
rows=13510 loops=1)
-> Nested Loop (cost=0.29..7386.57 rows=13510 width=11) (actual
time=0.104..1.235 rows=73 loops=1)
-> Seq Scan on country gcountry (cost=0.00..14.58 rows=258 width=27)
(actual time=0.012..0.126 rows=258 loops=1)
-> Index Scan using order_getxmladdresscountryid_fidx on "order"
torder (cost=0.29..27.89 rows=68 width=366) (actual time=0.003..0.003 rows=0
loops=258)
Index Cond: (g.getxmladdresscountryid(szxmladdress) =
gcountry.oid)
-> Index Scan using order_getxmladdresscountryid_fidx on "order" torder_1
(cost=0.29..139.47 rows=68 width=0) (actual time=0.013..12.840 rows=13437
loops=1)
Index Cond: (g.getxmladdresscountryid(szxmladdress) IS NULL)
Planning time: 1.104 ms
Execution time: 23.607 ms
A similar result was achieved by return a dummy ID from the function for
records without country.
Any idea why the functional index does not work in a LEFT JOIN query?
Thanks Klaus
Von: Jeff Janes [mailto:[email protected]]
Gesendet: Dienstag, 21. März 2017 08:22
An: Klaus P. Pieper
Betreff: Re: [GENERAL] Why is this functional index not used?
On Mon, Mar 20, 2017 at 1:25 PM, Klaus P. Pieper < <mailto:[email protected]>
[email protected]> wrote:
I played around with COST of the function between 1 and 20000 and with several
options on postgresql.conf without luck.
Why not more than 20000 ? The true value could be much higher than that.
Cheers,
Jeff
