Improve perfomance for index search ANY(ARRAY[]) condition with single item

Hello,

The problems I tried to solve here:
1. Improve perfomance for index search ANY(ARRAY[...]) condition with
single item
2. I saw tons of users code like: if len(array) == 1: sql +=
'{}'.format(array[0]) else: sql += 'ANY(ARRAY[{}])'.format(array)
So there will be less lines of code and it will be clearer.
3. Confusing moment that "IN" works well with single item, and
"ANY(ARRAY[])" doesn't without any real reason.

The problem was discussed on stackoverflow:
https://stackoverflow.com/questions/45061966/index-usage-with-single-item-anyarray

That's my first patch so I will be grateful for constructive criticism.
---------------------------------------------------------------------------

CREATE TABLE public.t (id serial, a integer, b integer);

INSERT INTO t(a, b)
SELECT round(random()*1000), round(random()*1000)
FROM generate_series(1, 1000000);

CREATE INDEX "i_1" ON public.t USING btree (a, b);
CREATE INDEX "i_2" ON public.t USING btree (b);

---------------------------------------------------------------------------

If "a = 50" in the first query, everything is ok, appropriate index "i_1"
is used:

SELECT * FROM t WHERE a = 50 ORDER BY b LIMIT 1

"Limit (cost=0.42..4.03 rows=1 width=12) (actual time=0.085..0.085 rows=1
loops=1)"
" Buffers: shared hit=1 read=3"
" -> Index Scan using i_1 on t (cost=0.42..4683.12 rows=1300 width=12)
(actual time=0.084..0.084 rows=1 loops=1)"
" Index Cond: (a = 50)"
" Buffers: shared hit=1 read=3"
"Planning time: 0.637 ms"
"Execution time: 0.114 ms"

---------------------------------------------------------------------------

With "a IN (50)" result is the same:

SELECT * FROM t WHERE a IN (50) ORDER BY b LIMIT 1

"Limit (cost=0.42..4.03 rows=1 width=12) (actual time=0.058..0.058 rows=1
loops=1)"
" Buffers: shared hit=4"
" -> Index Scan using i_1 on t (cost=0.42..4683.12 rows=1300 width=12)
(actual time=0.056..0.056 rows=1 loops=1)"
" Index Cond: (a = 50)"
" Buffers: shared hit=4"
"Planning time: 0.287 ms"
"Execution time: 0.105 ms"

---------------------------------------------------------------------------

The problem is when I try to use "a = ANY(ARRAY[50])". Wrong index "i_2" is
used instead of "i_1" and execution time becomes x25 longer:

SELECT * FROM t WHERE a = ANY(ARRAY[50]) ORDER BY b LIMIT 1

"Limit (cost=0.42..38.00 rows=1 width=12) (actual time=2.591..2.591 rows=1
loops=1)"
" Buffers: shared hit=491 read=4"
" -> Index Scan using i_2 on t (cost=0.42..48853.65 rows=1300 width=12)
(actual time=2.588..2.588 rows=1 loops=1)"
" Filter: (a = ANY ('{50}'::integer[]))"
" Rows Removed by Filter: 520"
" Buffers: shared hit=491 read=4"
"Planning time: 0.251 ms"
"Execution time: 2.627 ms"