[database-devel] Arrays and postgresql-simple

[Jason Dusek]

I have integrated Bas's type cache modifications with a more
robust parser and pretty printer, which handle quoted strings
and nested arrays.

  https://github.com/solidsnack/postgresql-simple/tree/arrays

In an earlier email, Leon P. Smith raised some concerns about
the types of arrays of arrays.

2012-07-30 21:00:34 UTC, leon.p.smith at gmail.com:
> Now, presumably, postgresql does actually support arrays of
> arrays, but I'm guessing that you need to create the array of
> array of string type before you can actually run this query...
>
> The reason I ask is that Bas van Dijk has done some work on
> adding support for arrays to postgresql-simple. And he's
> modified the type cache from TypeOID -> IO TypeName to become
> a TypeOID -> IO TypeInfo, where the TypeInfo type is defined
> as follows:
>
> data NamedOid = NamedOid { typoid  :: !PQ.Oid
>                          , typname :: !ByteString
>                          } deriving Show
>
> data TypeInfo = TypeInfo { typ     :: !NamedOid
>                          , typelem :: !(Maybe NamedOid)
>                          } deriving Show
>
> I think this is a perfectly reasonable first attempt, but I
> don't think it's correct, because I'm pretty sure that
> PostgreSQL does actually support arrays of arrays if you know
> what you are doing. So I think that TypeInfo needs to look
> something more like
>
> data TypeInfo = Plain { typ     :: !NamedOid }
>               | Array { typ     :: !NamedOid
>                       , typelem :: !TypeInfo }
>                 deriving Show
>
> The real issue here is one dealing with purity and effects: In
> some sense, Bas's first attempt carries the same information,
> but you might need to do some IO in order to retrieve it.
> (Though this assumes that one can query the type cache
> directly, which is something I should probably add anyway...)
> But the fromField method, which is is the most likely consumer
> of this information, isn't allowed to do IO. So I think we
> really want to change this type.

Because a Postgres array is multi-dimensional, it would stand to
reason that the Haskell types [Int4], [[Int4]] and [[[Int4]]]
would all map to _int4 in Postgres. The array parsing code
linked to above relies on that assumption.

--
Jason Dusek
pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B