Postgres 13

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.
-- Type resolution has to kick in **before** the array constructor is applied
-- as there is not "array of unknown"
-- The default type of a quoted string literal ("string constant") is text
-- So the resulting type is text[]
SELECT pg_typeof(ARRAY['1']);


pg_typeof


text[]


-- And there is no operator for integer[] && text[]:
SELECT '{1,2,3}'::int[] && ARRAY['1'];
ERROR:  operator does not exist: integer[] && text[]
LINE 2: SELECT '{1,2,3}'::int[] && ARRAY['1'];
                                ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.


-- An explicit cast can fix it.
-- Ideally on the element type, *before* forming the array:
SELECT '{1,2,3}'::int[] && ARRAY[int '1']
     , '{1,2,3}'::int[] && ARRAY['1'::integer]
-- But casting the array itself works, too:
     , '{1,2,3}'::int[] && ARRAY['1']::integer[]
     , '{1,2,3}'::int[] && ARRAY['1']::int[];


?column?
?column?
?column?
?column?


t
t
t
t


-- Without ARRAY constructor, type resolution can derive a fitting type from the implicit assignment in the expression:
SELECT 1 <> '1';  -- int is derived


?column?


f


-- If *typed values* are passed in, you need to cast either way
SELECT '{1,2,3}'::int[] && ARRAY[text '1'];  -- nothing to assume, incompatiple by definition
ERROR:  operator does not exist: integer[] && text[]
LINE 2: SELECT '{1,2,3}'::int[] && ARRAY[text '1'];  -- nothing to a...
                                ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.


-- If *typed values* are passed in, you need to cast either way
SELECT 1 <> text '1'; -- nothing to assume, incompatiple by definition
ERROR:  operator does not exist: integer <> text
LINE 2: SELECT 1 <> text '1'; -- nothing to assume, incompatiple by ...
                 ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.


-- Works with explicit cast like demonstrated above:
SELECT '{1,2,3}'::int[] && ARRAY[(text '1')::int], 1 <> (text '1')::int;


?column?
?column?


t
f