Prohibit DML queries in function bodies

The proposal is to prohibit INSERT, UPDATE, and DELETE statements in function bodies.

Currently these statements cannot appear in conditional expressions. More specifically, they are executed unconditionally, so it is semantically misleading to place them in a conditional branch that may or may not be executed - so this is forbidden. There’s a specialized form of these statements that allows them to be conditionally executed #1639.

However, functions don’t have the same unconditional execution policy and if these statements were to appear in functions, this could lead to unpredictable results, especially since function calls may be optimized by the query planner based on volatility and where they appear. In order to prevent unspecified behavior we currently choose to ban data manipulation inside function bodies.

[tailhook]

mutation can also write to the database and external state. Can neither be used in constraints nor in conditionals.

I would even say that we need a statement like CALL create_new_session() or MUTATE WITH create_new_session() and this is the only place where these functions can be used. Then these functions are as apparent as INSERT/UPDATE/DELETE statements and can be used in a same way (i.e. not a part of conditionals, …etc)

[CodesInChaos]

The simplest functions are pure, deterministic, do not access the database and always terminate normally. On top of that there are a bunch of additional capabilities a function could have:

1. Non determinism/read access to external state (randomness, current time, …)
2. Read access to the database
3. Write access to the database
4. Write access to external state
5. Throw an exception
6. Run forever

Functions used in constraints cannot access to database and not determinism is generally a bad idea in constraints.

Functions mutating cannot be optimized out and can only be used in certain places.

IMO you should encode at least enough of the above properties in the function to decide if the function can be used in constraints or conditionals.

The minimal design which achieves that would be three types of function:

• function (pure, deterministic, can throw exceptions and run forever. Can be used in constraints and conditionals.
• query can also read from the database and be read external state/be non deterministic. Can not be used in constraints but can be used in conditionals.
• mutation can also write to the database and external state. Can neither be used in constraints nor in conditionals.