Add immutable to cast functions

[tobyhede]

Given an encrypted column and an index defined as

CREATE INDEX encrypted_index ON encrypted (e eql_v2.encrypted_operator_class);

Indexes are not being used when the compared value was cast.

SELECT e::jsonb FROM encrypted WHERE e = '{"hm": "abc"}'::jsonb;
SELECT e::jsonb FROM encrypted WHERE e = '{"hm": "abc"}'::jsonb::encrypted;

Indexes are used when the value is the full literal tuple type

SELECT e::jsonb FROM encrypted WHERE e = '("{\"hm\": \"abc\"}")';

Adding the IMMUTABLE volatility category to the the type cast functions resolves the issue.

[yujiyokoo]

I don't understand why IMMUTABLE resolves the issue here 😅 but code LGTM

[tobyhede]

@yujiyokoo What I wish I knew earlier: https://www.postgresql.org/docs/current/sql-createindex.html

All functions and operators used in an index definition must be “immutable”, that is, their results must depend only on their arguments and never on any outside influence (such as the contents of another table or the current time). This restriction ensures that the behavior of the index is well-defined. To use a user-defined function in an index expression or WHERE clause, remember to mark the function immutable when you create it.