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From: jian he <jian.universality@gmail.com>
Date: Thu, 26 Mar 2026 16:59:56 +0800
Message-ID: 
 <CACJufxH_1EtEBMb0JvxaM3Gmnt33HYrS37m5eYjJ_OfBkMVFJg@mail.gmail.com>
Subject: Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User
 Functions
To: Peter Eisentraut <peter@eisentraut.org>
Cc: Corey Huinker <corey.huinker@gmail.com>, Amul Sul <sulamul@gmail.com>,
	Kirill Reshke <reshkekirill@gmail.com>,
 Vik Fearing <vik@postgresfriends.org>,
	Isaac Morland <isaac.morland@gmail.com>, pgsql-hackers@lists.postgresql.org
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Precedence: bulk

Currently, when a valid ErrorSaveContext is passed to state->escontext
(ExprState->escontext),
ExecInitExprRec is designed to compile the entire expression tree
using soft errors.
Consider the following example:
    create table t1(a text, b text, c int, d int8);
    create domain d2 as text check (value <> 'a');
    insert into t1(a, d) values('a', 2147483648);
    select cast (cast(d as int) as text default null on conversion
error) from t1;  -- queryA
    select cast (cast(a as d2) as text default null on conversion
error) from t1;   -- queryB
    select cast (cast(a as int) as text default null on conversion
error) from t1;  -- queryC

With V24, for queryC, ExecInitExprRec will compile the inner ``cast(a as int)``
error-safe way. This is just a subexpression of the main outer
cast(... as text default ...) node,
we may want the inner cast to be evaluated not error-safe.

To resolve this, we need to differentiate whether the specific nodes produced by
coerce_to_target_type->build_coercion_expression/coerce_to_domain
should be error-safe.
This means the resulting nodes (FuncExpr, ArrayCoerceExpr,
CoerceViaIO, CoerceToDomain) need a boolean flag.

We have already solved this for FuncExpr in [1] by adding an errorsafe field. We
now need to add a similar errorsafe boolean to CoerceViaIO and CoerceToDomain.

Therefore, for safe error evaluation: 1. ExecInitExprRec, ExprState->escontext
must be a valid ErrorSaveContext 2. The expression node (need a booleaan) to
tell ExecInitExprRec that compile this in an error safe manner.

[1]: https://postgr.es/m/CACJufxGbw9iNT8QVm4QD9cPFKnDnvDBQp7AGxkoqDa-JjzVXmg@mail.gmail.com

Summary: Making the above queryA, queryB, and queryC return NULL is
straightforward; however, forcing all of them to throw an error is non-trivial
because it involves compiling/evaluating some subexpressions in an
error-safe manner while
others are not.

What do you think?


--
jian
https://www.enterprisedb.com/