Debugging performance differences (and issues) between a query with a double-subquery, a single-subquery and an all inner-join statements - sql

Question

I have a complex business logic that requires me to perform a 2-levels nested query. The queries are generated by Django's ORM. At the bottom of the question I'll provide the queries as-is as well as a full EXPLAIN suitable to be viewed with PEV2, but in order to help readers understand better the question, I'll start with a more conceptual explanation.
This is how a very naive description of what we're doing looks like:
some_ids = get_id_based_on_some_conditions(*conditions*)
some_other_ids = get_some_other_ids_based_on_some_conditions_and_filtering_by_some_ids(*other_conditions*, some_ids)
results = get_results_based_on_even_more_conditions_and_filtering_by_some_other_ids(*another_set_of_conditions*, some_other_ids)
Translating the following pseudo-code to actual SQL using subqueries is quite easy. A straightforward translation becomes into the following pseudo-query:
select
foo,
bar
from
t1,
t2
where
condition1 = something and
condition2 in ( <---- first level subquery
select
id
from
t3
where
condition3 = another_something and
condition4 in ( <---- second level subquery
select
another_id
from
t4
where
condition5 = something_something and
condition6 = another_something_something
)
)
Since the query is taking a considerable amount of time (~0.6s) given the number of rows that it returns (a little bit over 9.000), I thought that it might help replacing the second level subquery with an inner join.
That, in fact, made the query even slower (now at ~1.7s). So I thought that maybe the planner didn't correctly understood what would happen with a subquery with an inner join inside and made some serious miscalculations / overestimations / underestimations, so I replaced the first level subquery with more inner joins, which led to even poorer results (now at ~10s).
I have been analysing the EXPLAINS of the queries for hours, and I can't figure out why using inner joins makes everything slower. I also don't know how to tell if my (currently) best query is actually the best I can get or if there are things that I'm not doing and that might speed it up.
So, the questions that I have are:
why are the inner joins slower than subqueries?
how can I tell if I'm doing everything possible in order to squeeze the maximum performance out of my database or if I'm missing something?
Actual queries and EXPLAINS as-is:
Query with 2-levels subqueries:
SELECT DISTINCT
"phdrug_phdrug"."id",
"phdrug_phdrug"."uuid",
"phdrug_phdrug"."default_description",
"phdrug_phdrug"."alternative_description",
"phdrug_phdrug"."ean",
"phdrug_phdrug"."mirror_ean",
"phdrug_phdrug"."parent_ean",
"phdrug_phdrug"."reg_num",
"phdrug_phdrug"."medika_code",
"phdrug_phdrug"."atc_iv",
"phdrug_phdrug"."product_type",
"phdrug_phdrug"."fraction",
"phdrug_phdrug"."active",
"phdrug_phdrug"."loyal",
"phdrug_phdrug"."patent",
"phdrug_phdrug"."chronics",
"phdrug_phdrug"."recipe",
"phdrug_phdrug"."deal",
"phdrug_phdrug"."specialized",
"phdrug_phdrug"."armored",
"phdrug_phdrug"."top_hight_speciality",
"phdrug_phdrug"."top_generic",
"phdrug_phdrug"."hight_speciality",
"phdrug_phdrug"."temp_8_15",
"phdrug_phdrug"."temp_15_25",
"phdrug_phdrug"."temp_2_8",
"phdrug_phdrug"."temp_less_15",
"phdrug_phdrug"."new",
"phdrug_phdrug"."mdk_internal_code",
"phdrug_phdrug"."mdk_single_id",
"phdrug_phdrug"."mdk_object_id",
"phdrug_phdrug"."is_from_mdk_db",
"phdrug_phdrug"."top",
"phdrug_phdrug"."laboratory_name",
"phdrug_phdrug"."laboratory_alternative_name",
"phdrug_phdrug"."imported",
"phdrug_phdrug"."imported_country",
"phdrug_phdrug"."laboratory_id",
"phdrug_phdrug"."specialty",
"phdrug_phdrug"."dimension_id",
"phdrug_phdrug"."featured",
"phdrug_phdrug"."top_ae_rank",
"phdrug_phdrug"."top_farma_rank"
FROM
"phdrug_phdrug"
INNER JOIN "monetary_drugprice" ON ( "phdrug_phdrug"."id" = "monetary_drugprice"."drug_id" )
INNER JOIN "phdrug_phdrugpicture" ON ( "phdrug_phdrug"."id" = "phdrug_phdrugpicture"."drug_id" )
WHERE
(
"monetary_drugprice"."id" IN (
SELECT
V0."id"
FROM
"monetary_drugprice" V0
WHERE
(
V0."pricelist_id" IN (
SELECT DISTINCT ON
( U0."id" ) U0."id"
FROM
"monetary_pricelist" U0
INNER JOIN "monetary_pricelistdestinations" U1 ON ( U0."id" = U1."pricelist_id" )
INNER JOIN "organization_organization" U2 ON ( U0."manager_id" = U2."id" )
INNER JOIN "courier_carrier_pricelists" U3 ON ( U0."id" = U3."pricelist_id" )
INNER JOIN "courier_carrier" U4 ON ( U3."carrier_id" = U4."id" )
INNER JOIN "courier_carrierdelivery" U5 ON ( U4."id" = U5."carrier_id" )
INNER JOIN "monetary_pricelistcountry" U6 ON ( U0."id" = U6."pricelist_id" )
WHERE
(
(
U0."expires" = FALSE
OR (
U0."expires" = TRUE
AND ( U0."datestart" AT TIME ZONE'UTC' ) :: DATE <= '2020-05-01'
AND ( U0."dateend" AT TIME ZONE'UTC' ) :: DATE >= '2020-05-01'
)
)
AND U0."active" = TRUE
AND U1."to_public" = TRUE
AND U2."organization_type" = 2
AND (
U5."dst_country" = 'MX'
OR U5."ignore_country_filter" = TRUE
)
AND U6."country" = 'MX'
AND U2."active" = TRUE
)
)
AND V0."stock" > 0
)
)
AND "phdrug_phdrug"."active" = TRUE
AND "phdrug_phdrugpicture"."is_main" = TRUE
)
ORDER BY
"phdrug_phdrug"."id" ASC,
"phdrug_phdrug"."default_description" ASC
Full explain: https://pastebin.com/jDy3FyKp
Query with 1-level subquery:
SELECT DISTINCT
"phdrug_phdrug"."id",
"phdrug_phdrug"."uuid",
"phdrug_phdrug"."default_description",
"phdrug_phdrug"."alternative_description",
"phdrug_phdrug"."ean",
"phdrug_phdrug"."mirror_ean",
"phdrug_phdrug"."parent_ean",
"phdrug_phdrug"."reg_num",
"phdrug_phdrug"."medika_code",
"phdrug_phdrug"."atc_iv",
"phdrug_phdrug"."product_type",
"phdrug_phdrug"."fraction",
"phdrug_phdrug"."active",
"phdrug_phdrug"."loyal",
"phdrug_phdrug"."patent",
"phdrug_phdrug"."chronics",
"phdrug_phdrug"."recipe",
"phdrug_phdrug"."deal",
"phdrug_phdrug"."specialized",
"phdrug_phdrug"."armored",
"phdrug_phdrug"."top_hight_speciality",
"phdrug_phdrug"."top_generic",
"phdrug_phdrug"."hight_speciality",
"phdrug_phdrug"."temp_8_15",
"phdrug_phdrug"."temp_15_25",
"phdrug_phdrug"."temp_2_8",
"phdrug_phdrug"."temp_less_15",
"phdrug_phdrug"."new",
"phdrug_phdrug"."mdk_internal_code",
"phdrug_phdrug"."mdk_single_id",
"phdrug_phdrug"."mdk_object_id",
"phdrug_phdrug"."is_from_mdk_db",
"phdrug_phdrug"."top",
"phdrug_phdrug"."laboratory_name",
"phdrug_phdrug"."laboratory_alternative_name",
"phdrug_phdrug"."imported",
"phdrug_phdrug"."imported_country",
"phdrug_phdrug"."laboratory_id",
"phdrug_phdrug"."specialty",
"phdrug_phdrug"."dimension_id",
"phdrug_phdrug"."featured",
"phdrug_phdrug"."top_ae_rank",
"phdrug_phdrug"."top_farma_rank"
FROM
"phdrug_phdrug"
INNER JOIN "monetary_drugprice" ON ( "phdrug_phdrug"."id" = "monetary_drugprice"."drug_id" )
INNER JOIN "phdrug_phdrugpicture" ON ( "phdrug_phdrug"."id" = "phdrug_phdrugpicture"."drug_id" )
WHERE
(
"monetary_drugprice"."id" IN (
SELECT
U0."id"
FROM
"monetary_drugprice" U0
INNER JOIN "monetary_pricelist" U1 ON ( U0."pricelist_id" = U1."id" )
INNER JOIN "monetary_pricelistdestinations" U2 ON ( U1."id" = U2."pricelist_id" )
INNER JOIN "organization_organization" U3 ON ( U1."manager_id" = U3."id" )
INNER JOIN "courier_carrier_pricelists" U4 ON ( U1."id" = U4."pricelist_id" )
INNER JOIN "courier_carrier" U5 ON ( U4."carrier_id" = U5."id" )
INNER JOIN "courier_carrierdelivery" U6 ON ( U5."id" = U6."carrier_id" )
INNER JOIN "monetary_pricelistcountry" U7 ON ( U1."id" = U7."pricelist_id" )
WHERE
(
(
U1."expires" = FALSE
OR (
U1."expires" = TRUE
AND ( U1."datestart" AT TIME ZONE'UTC' ) :: DATE <= '2020-05-01'
AND ( U1."dateend" AT TIME ZONE'UTC' ) :: DATE >= '2020-05-01'
)
)
AND U1."active" = TRUE
AND U2."to_public" = TRUE
AND U3."organization_type" = 2
AND (
U6."dst_country" = 'MX'
OR U6."ignore_country_filter" = TRUE
)
AND U7."country" = 'MX'
AND U3."active" = TRUE
AND U0."stock" > 0
)
)
AND "phdrug_phdrug"."active" = TRUE
AND "phdrug_phdrugpicture"."is_main" = TRUE
)
ORDER BY
"phdrug_phdrug"."id" ASC,
"phdrug_phdrug"."default_description" ASC
Full explain: https://pastebin.com/NidTZMxY
Query with only inner joins:
SELECT DISTINCT
"phdrug_phdrug"."id",
"phdrug_phdrug"."uuid",
"phdrug_phdrug"."default_description",
"phdrug_phdrug"."alternative_description",
"phdrug_phdrug"."ean",
"phdrug_phdrug"."mirror_ean",
"phdrug_phdrug"."parent_ean",
"phdrug_phdrug"."reg_num",
"phdrug_phdrug"."medika_code",
"phdrug_phdrug"."atc_iv",
"phdrug_phdrug"."product_type",
"phdrug_phdrug"."fraction",
"phdrug_phdrug"."active",
"phdrug_phdrug"."loyal",
"phdrug_phdrug"."patent",
"phdrug_phdrug"."chronics",
"phdrug_phdrug"."recipe",
"phdrug_phdrug"."deal",
"phdrug_phdrug"."specialized",
"phdrug_phdrug"."armored",
"phdrug_phdrug"."top_hight_speciality",
"phdrug_phdrug"."top_generic",
"phdrug_phdrug"."hight_speciality",
"phdrug_phdrug"."temp_8_15",
"phdrug_phdrug"."temp_15_25",
"phdrug_phdrug"."temp_2_8",
"phdrug_phdrug"."temp_less_15",
"phdrug_phdrug"."new",
"phdrug_phdrug"."mdk_internal_code",
"phdrug_phdrug"."mdk_single_id",
"phdrug_phdrug"."mdk_object_id",
"phdrug_phdrug"."is_from_mdk_db",
"phdrug_phdrug"."top",
"phdrug_phdrug"."laboratory_name",
"phdrug_phdrug"."laboratory_alternative_name",
"phdrug_phdrug"."imported",
"phdrug_phdrug"."imported_country",
"phdrug_phdrug"."laboratory_id",
"phdrug_phdrug"."specialty",
"phdrug_phdrug"."dimension_id",
"phdrug_phdrug"."featured",
"phdrug_phdrug"."top_ae_rank",
"phdrug_phdrug"."top_farma_rank"
FROM
"phdrug_phdrug"
INNER JOIN "monetary_drugprice" ON ( "phdrug_phdrug"."id" = "monetary_drugprice"."drug_id" )
INNER JOIN "monetary_pricelist" ON ( "monetary_drugprice"."pricelist_id" = "monetary_pricelist"."id" )
INNER JOIN "monetary_pricelistdestinations" ON ( "monetary_pricelist"."id" = "monetary_pricelistdestinations"."pricelist_id" )
INNER JOIN "organization_organization" ON ( "monetary_pricelist"."manager_id" = "organization_organization"."id" )
INNER JOIN "courier_carrier_pricelists" ON ( "monetary_pricelist"."id" = "courier_carrier_pricelists"."pricelist_id" )
INNER JOIN "courier_carrier" ON ( "courier_carrier_pricelists"."carrier_id" = "courier_carrier"."id" )
INNER JOIN "courier_carrierdelivery" ON ( "courier_carrier"."id" = "courier_carrierdelivery"."carrier_id" )
INNER JOIN "monetary_pricelistcountry" ON ( "monetary_pricelist"."id" = "monetary_pricelistcountry"."pricelist_id" )
INNER JOIN "phdrug_phdrugpicture" ON ( "phdrug_phdrug"."id" = "phdrug_phdrugpicture"."drug_id" )
WHERE
(
(
"monetary_pricelist"."expires" = FALSE
OR (
"monetary_pricelist"."expires" = TRUE
AND ( "monetary_pricelist"."datestart" AT TIME ZONE'UTC' ) :: DATE <= '2020-05-01'
AND ( "monetary_pricelist"."dateend" AT TIME ZONE'UTC' ) :: DATE >= '2020-05-01'
)
)
AND "monetary_pricelist"."active" = TRUE
AND "monetary_pricelistdestinations"."to_public" = TRUE
AND "organization_organization"."organization_type" = 2
AND (
"courier_carrierdelivery"."dst_country" = 'MX'
OR "courier_carrierdelivery"."ignore_country_filter" = TRUE
)
AND "monetary_pricelistcountry"."country" = 'MX'
AND "organization_organization"."active" = TRUE
AND "monetary_drugprice"."stock" > 0
AND "phdrug_phdrug"."active" = TRUE
AND "phdrug_phdrugpicture"."is_main" = TRUE
)
ORDER BY
"phdrug_phdrug"."id" ASC,
"phdrug_phdrug"."default_description" ASC
Full explain: https://pastebin.com/DaVztBuV

Answer 1

Troubleshooting at this level is difficult without seeing the database structure. I've had to write two different versions of the same script because the environments were different at the different sites.
Make sure the tables are properly indexed.
Make your subscript as part of the FROM statement and not the WHERE statement, unless its part of the IN clause.
Select *
from Table1 t1
left outer join (Select * from Table2) t2 on t1.field = t2.field
If its a large pull and/or large heavy used tables, then using temp tables will speed it as well. But it looks like your script is smaller and this is over kill.