Can you help me with this situation please??

I have a strange problem with a query where the planner only uses and index
if I use a constant value, but if I use a subquery it will prefer a seq

I have table "sample_table" with columns id serial primary key, and
int_flag, with an index on int_flag.
I inserted 240387 values with int_flag=1 and 1 value with int_flag=2

so the table has 240388 total rows, the last row of the table has int_flag=2

If I execute this query, the planner chooses the index:

explain (analyze ,verbose,buffers)
FROM sample_table
  int_flag = any((array[2])::int[])


Index Scan using index_sample_table_int_flag_ix on public.sample_table
(cost=0.42..39.86 rows=1 width=8) (actual time=0.023..0.023 *rows=1*
  Output: id

  Index Cond: (sample_table.int_flag = ANY ('{2}'::integer[]))

  Buffers: shared hit=28

Planning time: 0.087 ms

Execution time: 0.046 ms

but if I slightly change the query to:

explain (analyze ,verbose,buffers)
FROM sample_table
  int_flag = any((*select* array[2])::int[])

now postgres will do a seq scan.
I have run vacuum and analyze but the result is the same.


Seq Scan on public.sample_table  (cost=0.01..8843.74 rows=240388 width=8)
(actual time=44.993..44.995 *rows=1* loops=1)
  Output: id

  Filter: (sample_table.int_flag = ANY ($0))

  Rows Removed by Filter: *240387*

  Buffers: shared hit=3435

  InitPlan 1 (returns $0)

    ->  Result  (cost=0.00..0.01 rows=1 width=32) (actual time=0.002..0.003
rows=1 loops=1)
          Output: '{2}'::integer[]

Planning time: 0.092 ms

Execution time: 45.017 ms

I have created a SQL Fiddle to demonstrate the issue:


I suppose postgres prefers a seq scan because it treats the subquery as a
non-deterministic value while in the first case the planner has all the
values before hand???

I'm using PostgreSQL 9.6.6 on x86_64-pc-linux-gnu, compiled by gcc (Debian
4.9.2-10) 4.9.2, 64-bit

PS: This is a simplified query, the actual query will use another small
table to build the array with less than 100 values and sample_table can
have up to 5 millions entries. I have tried using a CTE with the array, but
it still will do a seq scan.


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