Make PgTypeInfo publicly-constructible to support custom types

[jplatte]

In PostgreSQL, custom types can be created with CREATE TYPE (e.g. enums). To interface with such types using sqlx, one would have to (amongst other things) implement HasSqlType<CustomType> for Postgres, which in turn requires being able to construct a PgTypeInfo. PgTypeInfo currently doesn't have a public constructor or all-public fields though.

[mehcode]

After some internal discussion we've decided to add something like:

PgTypeInfo::with_name("dsflsdjfl")
PgTypeInfo::with_oid(201012)

with_name will be added a bit later but will involve querying postgres at runtime for the OID ( and caching it ).

[jplatte]

will involve querying postgres at runtime for the OID

So the PgTypeInfo will basically become statful for this case? As in, you first construct it as "an unknown type with the name x" and that later becomes "the type with the OID _"?

Or will HasSqlType::type_info be changed so a database connection is available?

[mehcode]

In short, no. It'll probably internally be an enum between "Name" and "OID".

• The Arguments type (already) builds an array of TypeInfo objects during the .bind, .bind process.

• Just before running PREPARE ... for the statement we can query for the OID ( of any custom types ) and add it to a typename to OID cache on PgConnection.

[abonander]

• and then when decoding the DataRow for Postgres we can look up in that cache to get the original type name again, which we can check in RowIndex::try_get().

[jplatte]

Ah, that makes sense. Thanks for all the details. Would you be interested in a PR?

[abonander]

Adding PgTypeInfo::with_oid()? Please do! The rest of the stuff is gonna be a bit involved.

Keep in mind that the reason that we want to support types by name is that the type OID isn't going to be stable across databases for custom types, if stuff got added in a different order or it's a newer version with more types. However, OIDs should be sufficient for tinkering with custom types against a single database.

[mehcode]

Added PgTypeInfo::with_name on master