Postgres Planner "Inconsistency"?

Renzo Bertuzzi wrote:

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 scan.

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)
SELECT id
FROM sample_table
WHERE
int_flag = any((array[2])::int[])

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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 loops=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)
SELECT id
FROM sample_table
WHERE
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.

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
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 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.

In the second case, the optimizer does not think hard enough to figure out
that it actually could know that the InitPlan has a result of 2, and with
your real query it probably couldn't know for sure even if it tried hard.

So it has to come up with a plan without knowing what the search values will
be, and it chooses a sequential scan as the lesser evil, since it guesses
that it will have to retrieve most of the tuples anyway.

Maybe you can write your query as a join instead.

Yours,
Laurenz Albe