Re: let ALTER COLUMN SET DATA TYPE cope with trigger dependency

hi.

v2-0001, v2-0002 is the same as [1].
The parameters relOid and refRelOid are removed from the CreateTrigger function
and instead added to the CreateTrigStmt structure.

The main reason is to avoid repeated name lookups.
In fact, we already did this in CreateTriggerFiringOn, see below code snippet.
If the OID is valid, use it preferentially; otherwise, fall back to resolving
the relation via the RangeVar.

```
if (OidIsValid(relOid))
rel = table_open(relOid, ShareRowExclusiveLock);
else
rel = table_openrv(stmt->relation, ShareRowExclusiveLock);

if (OidIsValid(refRelOid))
{
LockRelationOid(refRelOid, AccessShareLock);
constrrelid = refRelOid;
}
else if (stmt->constrrel != NULL)
constrrelid = RangeVarGetRelid(stmt->constrrel, AccessShareLock,
false);
```

v2-0003: is the actual implementation.
The general workflow would be as follows: collect all affected triggers,
retrieve their definitions, drop the existing triggers, and then recreate them.
Any comments associated with the triggers will also be dropped and recreated.

[1]: https://postgr.es/m/CACJufxFu7Y4FhVkaKT2Kaj8ym2T5TcwN93cR_6h4x66iLrSZ-Q@mail.gmail.com

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