Index not used with = ANY() but used with IN

Table t has two indexes: create table t (a int, b int); create type int_pair as (a int, b int); create index t_row_idx on t (((a,b)::int_pair)); create index t_a_b_idx on t (a,b); insert into t (a,b) select i, i from generate_series(1, 100000) g(i) ; No index is used with the ANY operator: explain analyze select * from t where (a,b) = any(array[(1,1),(1,2)]) ; QUERY PLAN --------------------------------------------------------------------------------------------------- Seq Scan on t (cost=0.00..1693.00 rows=1000 width=8) (actual time=0.042..126.789 rows=1 loops=1) Filter: (ROW(a, b) = ANY (ARRAY[ROW(1, 1), ROW(1, 2)])) Rows Removed by Filter: 99999 Planning time: 0.122 ms Execution time: 126.836 ms But one of them is used with the IN operator: explain analyze select * from t where (a,b) in ((1,1),(1,2)) ; QUERY PLAN ------------------------------------------------------------------------------------------------------------------ Index Only Scan using t_a_b_idx on t (cost=0.29..8.32 rows=1 width=8) (actual time=0.028..0.029 rows=1 loops=1) Index Cond: (a = 1) Filter: ((b = 1) OR (b = 2)) Heap Fetches: 1 Planning time: 0.161 ms Execution time: 0.066 ms It uses the record index if the record is cast to the correct type: explain analyze select * from t where (a,b)::int_pair = any(array[row(1,1),row(1,2)]) ; QUERY PLAN -------------------------------------------------------------------------------------------------------------- Index Scan using t_row_idx on t (cost=0.42..12.87 rows=2 width=8) (actual time=0.106..0.126 rows=1 loops=1) Index Cond: (ROW(a, b)::int_pair = ANY (ARRAY[ROW(1, 1), ROW(1, 2)])) Planning time: 0.208 ms Execution time: 0.203 ms Why doesn't the planner use the non record index for the ANY operator as it uses it for the IN operator?