Re: query planner not using index, instead using squential scan

by setting enable_sequence_scan=OFF, the query execution time seems to
have slowed down even though the index is being used for left join of
order_offer_map

Hash Left Join (cost=72639.74..8176118.25 rows=19276467 width=293) (actual
time=858.853..3166.994 rows=230222 loops=1)
Hash Cond: (ro.order_id = oom.order_id)
-> Hash Join (cost=1947.33..2053190.95 rows=19276467 width=211) (actual
time=20.550..462.303 rows=230222 loops=1)
Hash Cond: (b.city_id = c.city_id)
-> Hash Join (cost=1937.65..1998751.06 rows=19276467 width=190)
(actual time=20.523..399.979 rows=230222 loops=1)
Hash Cond: (b.restaurant_id = r.restaurant_id)
-> Hash Join (cost=1596.61..1947784.40 rows=19276467
width=190) (actual time=19.047..339.984 rows=230222 loops=1)
Hash Cond: (ro.branch_id = b.branch_id)
-> Nested Loop (cost=0.56..1895577.38 rows=19276467
width=108) (actual time=0.032..240.278 rows=230222 loops=1)
-> Function Scan on start_date (cost=0.00..0.01
rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=1)
-> Index Scan using "idx$$_00010001" on
restaurant_order ro (cost=0.56..1702812.70 rows=19276467 width=108)
(actual time=0.025..117.525 rows=230222 loops=1)
Index Cond: ((date_time >=
start_date.start_date) AND (date_time <= '2021-06-05
21:09:50.161463+00'::timestamp with time zone))
-> Hash (cost=1334.19..1334.19 rows=20949 width=90)
(actual time=18.969..18.969 rows=20949 loops=1)
Buckets: 32768 Batches: 1 Memory Usage: 2758kB
-> Index Scan using "branch_idx$$_274b0038" on
branch b (cost=0.29..1334.19 rows=20949 width=90) (actual
time=0.008..14.371 rows=20949 loops=1)
-> Hash (cost=233.42..233.42 rows=8609 width=8) (actual
time=1.450..1.451 rows=8609 loops=1)
Buckets: 16384 Batches: 1 Memory Usage: 465kB
-> Index Only Scan using "restaurant_idx$$_274b003d"
on restaurant r (cost=0.29..233.42 rows=8609 width=8) (actual
time=0.011..0.660 rows=8609 loops=1)
Heap Fetches: 0
-> Hash (cost=8.98..8.98 rows=56 width=29) (actual
time=0.021..0.021 rows=56 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 12kB
-> Index Only Scan using "city_idx$$_274b0022" on city c
(cost=0.14..8.98 rows=56 width=29) (actual time=0.004..0.010 rows=56
loops=1)
Heap Fetches: 0
-> Hash (cost=54779.81..54779.81 rows=1273009 width=15) (actual
time=836.132..836.133 rows=1273009 loops=1)
Buckets: 2097152 Batches: 1 Memory Usage: 81629kB
-> Index Scan Backward using order_offer_map_order_id on
order_offer_map oom (cost=0.43..54779.81 rows=1273009 width=15) (actual
time=0.010..578.226 rows=1273009 loops=1)
Planning Time: 1.229 ms
Execution Time: 3183.248 ms

On Sat, Jun 5, 2021 at 10:52 PM Vijaykumar Jain <
vijaykumarjain(dot)github(at)gmail(dot)com> wrote:

> thanks Tom.
>
> I was trying to simulate some scenarios to be able to explain how the plan
> would change with/without
> *Rows Removed by Filter: 73969 * -- by using a different/correct index.
>
> postgres=# \d t
> Table "public.t"
> Column | Type | Collation | Nullable | Default
> ------------+-----------------------------+-----------+----------+---------
> id | integer | | not null |
> created_on | timestamp without time zone | | |
> col1 | text | | |
> Indexes:
> "t_pkey" PRIMARY KEY, btree (id)
> "t_created_on_idx" btree (created_on) WHERE col1 = 'a'::text ---
> useless index as all rows have col1 = 'a', but to attempt lossy case
> "t_created_on_idx1" btree (created_on)
> Referenced by:
> TABLE "t1" CONSTRAINT "t1_id_fkey" FOREIGN KEY (id) REFERENCES t(id)
>
> postgres=# \d t1
> Table "public.t1"
> Column | Type | Collation | Nullable | Default
> --------+---------+-----------+----------+---------
> t1_id | integer | | not null |
> id | integer | | |
> col2 | text | | |
> Indexes:
> "t1_pkey" PRIMARY KEY, btree (t1_id)
> Foreign-key constraints:
> "t1_id_fkey" FOREIGN KEY (id) REFERENCES t(id)
>
>
>
> postgres=# update t set col1 = 'a';
> UPDATE 1000
>
> postgres=# explain analyze select 1 from t join t1 on (t.id = t1.id)
> where created_on = '2021-06-01 12:48:45.141123';
> QUERY PLAN
>
> --------------------------------------------------------------------------------------------------------
> Hash Join (cost=37.01..39.28 rows=1 width=4) (actual time=0.124..0.125
> rows=0 loops=1)
> Hash Cond: (t1.id = t.id)
> -> Seq Scan on t1 (cost=0.00..2.00 rows=100 width=4) (actual
> time=0.004..0.008 rows=100 loops=1)
> -> Hash (cost=37.00..37.00 rows=1 width=4) (actual time=0.109..0.109
> rows=1 loops=1)
> Buckets: 1024 Batches: 1 Memory Usage: 9kB
> -> Seq Scan on t (cost=0.00..37.00 rows=1 width=4) (actual
> time=0.058..0.107 rows=1 loops=1)
> Filter: (created_on = '2021-06-01
> 12:48:45.141123'::timestamp without time zone)
> *Rows Removed by Filter: 999 --- as no useful
> index, t_created_on_idx will fetch all pages and then remove rows from
> them, expensive*
> Planning Time: 0.111 ms
> Execution Time: 0.162 ms
> (10 rows)
>
>
> postgres=# explain analyze select 1 from t join t1 on (t.id = t1.id)
> where created_on = '2021-06-01 12:48:45.141123';
> QUERY PLAN
>
> ---------------------------------------------------------------------------------------------------------------------------------
> Hash Join (cost=8.32..33.47 rows=1 width=4) (actual time=0.025..0.026
> rows=0 loops=1)
> Hash Cond: (t1.id = t.id)
> -> Seq Scan on t1 (cost=0.00..22.00 rows=1200 width=4) (actual
> time=0.009..0.009 rows=1 loops=1)
> -> Hash (cost=8.31..8.31 rows=1 width=4) (actual time=0.014..0.014
> rows=0 loops=1)
> Buckets: 1024 Batches: 1 Memory Usage: 8kB
> -> Index Scan using t_created_on_idx1 on t (cost=0.29..8.31
> rows=1 width=4) (actual time=0.014..0.014 rows=0 loops=1)
> Index Cond: (created_on = '2021-06-01
> 12:48:45.141123'::timestamp without time zone) -- *exact match using
> btree index, *
> Planning Time: 0.255 ms
> Execution Time: 0.071 ms
> (9 rows)
>
>
> but from Ayub's plan, the number of rows fetched are a lot, but is also
> removing rows post index scan.
> if that can be improved with a btree index that does not filter unwanted
> rows, the run may be faster ?
> but i guess if there are 156k rows, planner would a have found a win in
> seq scan.
>
> Ayub,
> just for the sake of understanding,
>
> can you run the query using
>
> postgres=# set enable_seqscan TO 0;
> SET
> postgres=# -- explain analyze <run the query>
>
> postgres=# set enable_seqscan TO 1;
> SET
>
>
> On Sun, 6 Jun 2021 at 00:59, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Ayub Khan <ayub(dot)hp(at)gmail(dot)com> writes:
>> > could someone clarify why the LEFT JOIN order_offer_map oom using
>> > (order_id) in the below query is using sequential scan instead of
>> > using index on order_id which is defined in order_offer_map table.
>>
>> Probably because it estimates the hash join to restaurant_order is
>> faster than a nestloop join would be. I think it's likely right.
>> You'd need very optimistic assumptions about the cost of an
>> individual index probe into order_offer_map to conclude that 156K
>> of them would be faster than the 476ms that are being spent here
>> to read order_offer_map and join it to the result of the
>> indexscan on restaurant_order.
>>
>> If, indeed, that *is* faster on your hardware, you might want
>> to dial down random_page_cost to get more-relevant estimates.
>>
>> regards, tom lane
>>
>>
>>
>
> --
> Thanks,
> Vijay
> Mumbai, India
>

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