bad plan

I posted this on hackers, but I had to post it here.

===================================================================================================================================
Hi all,
running a 7.4.5 engine, I'm facing this bad plan:

empdb=# explain analyze SELECT name,url,descr,request_status,url_status,size_mb,estimated_start,request_time_stamp
empdb-# FROM v_sc_user_request
empdb-# WHERE
empdb-# login = 'babinow1'
empdb-# LIMIT 10 ;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=1716.38..1716.39 rows=1 width=232) (actual time=52847.239..52847.322 rows=10 loops=1)
-> Subquery Scan v_sc_user_request (cost=1716.38..1716.39 rows=1 width=232) (actual time=52847.234..52847.301 rows=10 loops=1)
-> Sort (cost=1716.38..1716.39 rows=1 width=201) (actual time=52847.219..52847.227 rows=10 loops=1)
Sort Key: sr.id_sat_request
-> Nested Loop Left Join (cost=1478.82..1716.37 rows=1 width=201) (actual time=3254.483..52847.064 rows=31 loops=1)
Join Filter: ("outer".id_package = "inner".id_package)
-> Nested Loop (cost=493.09..691.55 rows=1 width=193) (actual time=347.665..940.582 rows=31 loops=1)
-> Nested Loop (cost=493.09..688.49 rows=1 width=40) (actual time=331.446..505.628 rows=31 loops=1)
Join Filter: ("inner".id_user = "outer".id_user)
-> Index Scan using user_login_login_key on user_login ul (cost=0.00..4.00 rows=2 width=16) (actual time=12.065..12.071 rows=1 loops=1)
Index Cond: ((login)::text = 'babinow1'::text)
-> Materialize (cost=493.09..531.37 rows=7656 width=28) (actual time=167.654..481.813 rows=8363 loops=1)
-> Seq Scan on sat_request sr (cost=0.00..493.09 rows=7656 width=28) (actual time=167.644..467.344 rows=8363 loops=1)
Filter: (request_time > (now() - '1 mon'::interval))
-> Index Scan using url_pkey on url u (cost=0.00..3.05 rows=1 width=161) (actual time=13.994..14.000 rows=1 loops=31)
Index Cond: ("outer".id_url = u.id_url)
-> Subquery Scan vsp (cost=985.73..1016.53 rows=1103 width=12) (actual time=25.328..1668.754 rows=493 loops=31)
-> Merge Join (cost=985.73..1011.01 rows=1103 width=130) (actual time=25.321..1666.666 rows=493 loops=31)
Merge Cond: ("outer".id_program = "inner".id_program)
-> Sort (cost=20.74..20.97 rows=93 width=19) (actual time=0.385..0.431 rows=47 loops=31)
Sort Key: programs.id_program
-> Seq Scan on programs (cost=0.00..17.70 rows=93 width=19) (actual time=0.022..11.709 rows=48 loops=1)
Filter: (id_program <> 0)
-> Sort (cost=964.99..967.75 rows=1102 width=115) (actual time=14.592..15.218 rows=493 loops=31)
Sort Key: sequences.id_program
-> Merge Join (cost=696.16..909.31 rows=1102 width=115) (actual time=79.717..451.495 rows=493 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Merge Left Join (cost=0.00..186.59 rows=1229 width=103) (actual time=0.101..366.854 rows=1247 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Index Scan using packages_pkey on packages p (cost=0.00..131.04 rows=1229 width=103) (actual time=0.048..163.503 rows=1247 loops=1)
-> Index Scan using package_security_id_package_key on package_security ps (cost=0.00..46.83 rows=855 width=4) (actual time=0.022..178.599 rows=879 loops=1)
-> Sort (cost=696.16..705.69 rows=3812 width=16) (actual time=79.582..79.968 rows=493 loops=1)
Sort Key: sequences.id_package
-> Seq Scan on sequences (cost=0.00..469.42 rows=3812 width=16) (actual time=0.012..78.863 rows=493 loops=1)
Filter: (estimated_start IS NOT NULL)
Total runtime: 52878.516 ms
(36 rows)

Disabling the nestloop then the execution time become more affordable:

empdb=# set enable_nestloop = false;
SET
empdb=# explain analyze SELECT name,url,descr,request_status,url_status,size_mb,estimated_start,request_time_stamp
empdb-# FROM v_sc_user_request
empdb-# WHERE
empdb-# login = 'babinow1'
empdb-# LIMIT 10 ;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=4467.64..4467.65 rows=1 width=232) (actual time=7091.233..7091.289 rows=10 loops=1)
-> Subquery Scan v_sc_user_request (cost=4467.64..4467.65 rows=1 width=232) (actual time=7091.228..7091.272 rows=10 loops=1)
-> Sort (cost=4467.64..4467.64 rows=1 width=201) (actual time=7091.216..7091.221 rows=10 loops=1)
Sort Key: sr.id_sat_request
-> Merge Left Join (cost=4462.07..4467.63 rows=1 width=201) (actual time=6377.732..7091.067 rows=31 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Sort (cost=3389.81..3389.81 rows=1 width=193) (actual time=1338.759..1338.814 rows=31 loops=1)
Sort Key: sr.id_package
-> Merge Join (cost=3285.05..3389.80 rows=1 width=193) (actual time=1318.877..1338.651 rows=31 loops=1)
Merge Cond: ("outer".id_url = "inner".id_url)
-> Sort (cost=1029.26..1029.26 rows=1 width=40) (actual time=703.085..703.113 rows=31 loops=1)
Sort Key: sr.id_url
-> Merge Join (cost=991.00..1029.25 rows=1 width=40) (actual time=702.740..702.984 rows=31 loops=1)
Merge Cond: ("outer".id_user = "inner".id_user)
-> Sort (cost=986.99..1006.13 rows=7656 width=28) (actual time=648.559..655.302 rows=8041 loops=1)
Sort Key: sr.id_user
-> Seq Scan on sat_request sr (cost=0.00..493.09 rows=7656 width=28) (actual time=201.968..614.631 rows=8363 loops=1)
Filter: (request_time > (now() - '1 mon'::interval))
-> Sort (cost=4.01..4.02 rows=2 width=16) (actual time=35.252..35.282 rows=1 loops=1)
Sort Key: ul.id_user
-> Index Scan using user_login_login_key on user_login ul (cost=0.00..4.00 rows=2 width=16) (actual time=35.214..35.221 rows=1 loops=1)
Index Cond: ((login)::text = 'babinow1'::text)
-> Sort (cost=2255.79..2308.95 rows=21264 width=161) (actual time=587.664..602.490 rows=21250 loops=1)
Sort Key: u.id_url
-> Seq Scan on url u (cost=0.00..727.32 rows=21264 width=161) (actual time=0.026..418.586 rows=21264 loops=1)
-> Sort (cost=1072.27..1075.03 rows=1103 width=12) (actual time=5015.761..5016.092 rows=493 loops=1)
Sort Key: vsp.id_package
-> Subquery Scan vsp (cost=985.73..1016.53 rows=1103 width=12) (actual time=898.876..5014.570 rows=494 loops=1)
-> Merge Join (cost=985.73..1011.01 rows=1103 width=130) (actual time=898.869..5011.954 rows=494 loops=1)
Merge Cond: ("outer".id_program = "inner".id_program)
-> Sort (cost=20.74..20.97 rows=93 width=19) (actual time=29.669..29.708 rows=47 loops=1)
Sort Key: programs.id_program
-> Seq Scan on programs (cost=0.00..17.70 rows=93 width=19) (actual time=0.035..29.525 rows=48 loops=1)
Filter: (id_program <> 0)
-> Sort (cost=964.99..967.75 rows=1102 width=115) (actual time=868.619..869.286 rows=494 loops=1)
Sort Key: sequences.id_program
-> Merge Join (cost=696.16..909.31 rows=1102 width=115) (actual time=44.820..867.649 rows=494 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Merge Left Join (cost=0.00..186.59 rows=1229 width=103) (actual time=19.563..835.352 rows=1248 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Index Scan using packages_pkey on packages p (cost=0.00..131.04 rows=1229 width=103) (actual time=12.796..457.520 rows=1248 loops=1)
-> Index Scan using package_security_id_package_key on package_security ps (cost=0.00..46.83 rows=855 width=4) (actual time=6.703..283.944 rows=879 loops=1)
-> Sort (cost=696.16..705.69 rows=3812 width=16) (actual time=25.222..25.705 rows=494 loops=1)
Sort Key: sequences.id_package
-> Seq Scan on sequences (cost=0.00..469.42 rows=3812 width=16) (actual time=0.017..24.412 rows=494 loops=1)
Filter: (estimated_start IS NOT NULL)
Total runtime: 7104.946 ms
(47 rows)

May I know wich parameter may I tune in order to avoid to "disable" the nested loop ?
===================================================================================================================================

I tried to reduce the runtime cost adding a new column on sat_request ( expired boolean ) in order to
have a better row extimation ( I used a partial index ) but nothing changed.

I finally was able to reduce the cost putting ( just for this query ):
set cpu_tuple_cost = 0.07

is it a resonable value ?

empdb=# set cpu_tuple_cost = 0.07;
SET
empdb=# explain analyze select *
empdb-# from v_sc_user_request
empdb-# where login = 'babinow1';
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Subquery Scan v_sc_user_request (cost=1978.23..1978.30 rows=1 width=364) (actual time=1612.719..1613.064 rows=31 loops=1)
-> Sort (cost=1978.23..1978.23 rows=1 width=201) (actual time=1612.700..1612.728 rows=31 loops=1)
Sort Key: sr.id_sat_request
-> Merge Left Join (cost=1974.05..1978.22 rows=1 width=201) (actual time=1537.343..1612.565 rows=31 loops=1)
Merge Cond: ("outer".id_package = "inner".id_package)
-> Sort (cost=887.92..887.93 rows=1 width=193) (actual time=475.924..476.020 rows=31 loops=1)
Sort Key: sr.id_package
-> Nested Loop (cost=4.07..887.91 rows=1 width=193) (actual time=145.782..475.851 rows=31 loops=1)
-> Hash Join (cost=4.07..884.65 rows=1 width=40) (actual time=139.816..464.678 rows=31 loops=1)
Hash Cond: ("outer".id_user = "inner".id_user)
-> Index Scan using idx_sat_request_expired on sat_request sr (cost=0.00..838.69 rows=8363 width=28) (actual time=19.696..443.702 rows=8460 loops=1)
Index Cond: (expired = false)
-> Hash (cost=4.07..4.07 rows=2 width=16) (actual time=11.779..11.779 rows=0 loops=1)
-> Index Scan using user_login_login_key on user_login ul (cost=0.00..4.07 rows=2 width=16) (actual time=11.725..11.732 rows=1 loops=1)
Index Cond: ((login)::text = 'babinow1'::text)
-> Index Scan using url_pkey on url u (cost=0.00..3.19 rows=1 width=161) (actual time=0.345..0.347 rows=1 loops=31)
Index Cond: ("outer".id_url = u.id_url)
-> Sort (cost=1086.13..1088.16 rows=813 width=12) (actual time=1060.374..1060.622 rows=390 loops=1)
Sort Key: vsp.id_package
-> Subquery Scan vsp (cost=676.18..1046.83 rows=813 width=12) (actual time=625.645..1059.388 rows=480 loops=1)
-> Hash Join (cost=676.18..989.92 rows=813 width=131) (actual time=625.637..1057.105 rows=480 loops=1)
Hash Cond: ("outer".id_package = "inner".id_package)
-> Hash Left Join (cost=79.67..302.87 rows=1341 width=104) (actual time=4.336..18.549 rows=1342 loops=1)
Hash Cond: ("outer".id_package = "inner".id_package)
-> Seq Scan on packages p (cost=0.00..145.87 rows=1341 width=104) (actual time=0.007..3.357 rows=1342 loops=1)
-> Hash (cost=77.27..77.27 rows=961 width=4) (actual time=3.685..3.685 rows=0 loops=1)
-> Seq Scan on package_security ps (cost=0.00..77.27 rows=961 width=4) (actual time=0.016..2.175 rows=974 loops=1)
-> Hash (cost=594.48..594.48 rows=813 width=31) (actual time=620.397..620.397 rows=0 loops=1)
-> Hash Join (cost=20.60..594.48 rows=813 width=31) (actual time=38.307..619.406 rows=480 loops=1)
Hash Cond: ("outer".id_program = "inner".id_program)
-> Seq Scan on sequences (cost=0.00..512.82 rows=830 width=16) (actual time=16.858..595.262 rows=480 loops=1)
Filter: (estimated_start IS NOT NULL)
-> Hash (cost=20.48..20.48 rows=47 width=19) (actual time=21.093..21.093 rows=0 loops=1)
-> Seq Scan on programs (cost=0.00..20.48 rows=47 width=19) (actual time=9.369..20.980 rows=48 loops=1)
Filter: (id_program <> 0)
Total runtime: 1614.123 ms
(36 rows)

Regards
Gaetano Mendola