Try convert

[robozmey]

TRY_CONVERT

We want function what casts value to type or returns NULL.

In this PR there are TRY_CONVERT as extension

Uses "soft" error handling (ereturn & safe calls) from #22

res = OidFunctionCall3Safe(funcId, value, typmod, true, &escontext);

if (escontext.error_occurred) {
*is_failed = true;
}

Need to rewrite some cast & in/out functions to ereturn() instead of ereport()

if (arg1 < SHRT_MIN || arg1 > SHRT_MAX)
		ereturn(fcinfo->context, 0,
				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
				 errmsg("smallint out of range")));

Beside we can catch erros by PG_TRY(); using #define USE_PG_TRY_CATCH

Type	Soft handle	PG_TRY handle
numric(int, float, numeric)	⬜️	✅
time/date(time(tz),timestamp(tz),date, interval,abstime,reltime)	⬜️	✅
character(text,char,varchar)	⬜️	✅
json, jsonb	⬜️	✅
xml	⬜️	⬜️
complex	⬜️	⬜️
point	⬜️	✅
money	⬜️	✅
network(inet,cidr,macaddr)	⬜️	✅
bit string(bit, varbit)	⬜️	✅
regtype	⬜️	✅
uuid	⬜️	✅

Cast features	Supports?
pg_cast	✅
cast via IO	✅
array	⬜️
typmod	✅
domain	⬜️

! No domain convert (source_type ---> base_type -X-> target_type)

! REPLACE FlushErrorState()

Due polymorphism have strange signature (some_type, anyelement) -> anyelement

Second parameter is DEFAULT_VALUE::TARGET_TYPE

TRY_CONVERT('42'::text, NULL::int2) -- returns 42::int2
TRY_CONVERT('42d'::text, NULL::int2) -- returns NULL::int2
TRY_CONVERT('42d'::text, 1234::int2) -- returns 1234::int2

We cannot have (any1, any2) -> any2 signature with different anyelement types, but we can create multiple (_, any) -> any functions in contrib/try_convert/try_convert--1.0.sql or use select add_type_for_try_convert('date'::regtype); to add types.

In init we can add all supported types, but with add_type_for_try_convert user can add whatever type he wants

SQL implementation of TRY_CONVERT

Standard implementation of try_convert on psql. Works only when have IO functions.

T1 -> text -> T1 -> T2

pgbench says sql version is 30x slower then standard CAST

!!! does not understand 'typmod's (char and char(10) are same)

CREATE OR REPLACE FUNCTION try_convert_by_sql(_in text, INOUT _out ANYELEMENT, source_type text default 'text')
  LANGUAGE plpgsql AS
$func$
BEGIN
   EXECUTE format('SELECT (%L::%s)::%s', $1, source_type, pg_typeof(_out))
   INTO  _out;
EXCEPTION WHEN others THEN
   -- do nothing: _out already carries default
END
$func$;

TESTS

!!! ADD Test what out try_convert_by_sql is works correctly

contrib/try_convert/generate_tests.py

Test category	Supports?
pg_cast	✅
to text	✅
from text	✅
from corrupted text	⬜️
with defaults	✅
1 million via func	✅
1 million via IO	✅
1 million via func with errors	✅
1 million via IO with errors	✅
nested casts	✅
nested casts with errors	⬜️(?)

BENCHMARK

Here are some reminders before you submit the pull request

• Add tests for the change
• Document changes
• Communicate in the mailing list if needed
• Pass make installcheck
• Review a PR in return to support the community

[reshke]

remove generated data from git

[reshke]

ABI change is not allowed without pg_upgrade

[reshke]

This ABI change may lead to numerous failures, both instant(when gpstart) and deferred (executing query with view).
You need to reuse existing fields within TypeCast struct.
One option is to prepend typeName with __opengpdb_trycast__ prefix

[reshke]

This design is highly dubious.
I would like a simpler approach, without try/catch and plan interceptions at all:

1. Take InputFunctionCallSafe from pg https://git.postgresql.org/cgit/postgresql.git/commit/?id=d9f7f5d32f201bec61fef8104aafcb77cecb4dcb
2. call it withing our try_convert function.
3. p.2 means no syntax changes, wee can define try_convert in extension, and then CREATE EXTENSION.