Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.
This work builds upon the Error-safe User Functions work currently ongoing.
The proposed changes are as follows:
CAST(expr AS typename)
continues to behave as before.
CAST(expr AS typename ERROR ON ERROR)
has the identical behavior as the unadorned CAST() above.
CAST(expr AS typename NULL ON ERROR)
will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.
CAST(expr AS typename DEFAULT expr2 ON ERROR)
will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.
There is an additional FORMAT parameter that I have not yet implemented, my understanding is that it is largely intended for DATE/TIME field conversions, but others are certainly possible.
CAST(expr AS typename FORMAT fmt DEFAULT expr2 ON ERROR)
What is currently working:
- Any scalar expression that can be evaluated at parse time. These tests from cast.sql all currently work:
VALUES (CAST('error' AS integer));
VALUES (CAST('error' AS integer ERROR ON ERROR));
VALUES (CAST('error' AS integer NULL ON ERROR));
VALUES (CAST('error' AS integer DEFAULT 42 ON ERROR));
SELECT CAST('{123,abc,456}' AS integer[] DEFAULT '{-789}' ON ERROR) as array_test1;
- Scalar values evaluated at runtime.
CREATE TEMPORARY TABLE t(t text);
INSERT INTO t VALUES ('a'), ('1'), ('b'), (2);
SELECT CAST(t.t AS integer DEFAULT -1 ON ERROR) AS foo FROM t;
foo
-----
-1
1
-1
2
(4 rows)
Along the way, I made a few design decisions, each of which is up for debate:
First, I created OidInputFunctionCallSafe, which is to OidInputFunctionCall what InputFunctionCallSafe is to InputFunctionCall. Given that the only place I ended up using it was stringTypeDatumSafe(), it may be possible to just move that code inside stringTypeDatumSafe.
Next, I had a need for FuncExpr, CoerceViaIO, and ArrayCoerce to all report if their expr argument failed, and if not, just past the evaluation of expr2. Rather than duplicate this logic in several places, I chose instead to modify CoalesceExpr to allow for an error-test mode in addition to its default null-test mode, and then to provide this altered node with two expressions, the first being the error-safe typecast of expr and the second being the non-error-safe typecast of expr2.
I still don't have array-to-array casts working, as the changed I would likely need to make to ArrayCoerce get somewhat invasive, so this seemed like a good time to post my work so far and solicit some feedback beyond what I've already been getting from Jeff Davis and Michael Paquier.
I've sidestepped domains as well for the time being as well as avoiding JIT issues entirely.
No documentation is currently prepared. All but one of the regression test queries work, the one that is currently failing is:
SELECT CAST('{234,def,567}'::text[] AS integer[] DEFAULT '{-1011}' ON ERROR) as array_test2;
Other quirks:
- an unaliased CAST ON DEFAULT will return the column name of "coalesce", which internally is true, but obviously would be quite confusing to a user.
As a side observation, I noticed that the optimizer already tries to resolve expressions based on constants and to collapse expression trees where possible, which makes me wonder if the work done to do the same in transformTypeCast/ and coerce_to_target_type and coerce_type isn't also wasted.
