[Haskell-cafe] Announce: HDBI-1.3 and friends

Wed Oct 16 13:16:39 UTC 2013

If I understand correctly you just want to re-execute the same query
multiple times. There is method `reset` in HDBI already. Just call `reset`
and statement return to it's initial state just after `prepare`, then
execute `fetchAll` to get the result. Or just create new statement with
`prepare` using the same query string or getting it from old statement with
`originalQuery`.

2013/10/16 Gauthier Segay <gauthier.segay at gmail.com>

> > use `runFetchAll` it will return constructed Seq with results. But I
> would use conduits and conduit `selectAll`
> http://hackage.haskell.org/package/hdbi-conduit-1.3.0/docs/Data-Conduit-HDBI.html.
> Conduits are realy simple and effective.
>
> I'm unsure I expressed the question properly, multiple results (rows)
> is definitely supported but multiple resultset does not seem to be.
>
> There are occurrences where you issue multiple select statements in a
> single roundtrip to the database, each select with potentially
> different row layout.
>
> After reading your answer, I actually tried (hdbc) calling the
> fetchRows function several times on Statement but it won't return
> anything past the first resultset.
>
> It seems the .net NextResults() approach is good because it let's you
> check whether or not there is a next resultset to fetch rows from, I
> think it would be necessary to have a similar approach if this is
> going to be supported.
>
>
> On Wed, Oct 16, 2013 at 6:03 AM, Alexey Uimanov <s9gf4ult at gmail.com>
> wrote:
> >> ability to use named parameters
> >
> > Yes. I dont know when It will be done, but it is on github issues already
> > https://github.com/s9gf4ult/hdbi/issues/3 .
> >
> >> fetching multiple results returned from single statement
> >
> > use `runFetchAll` it will return constructed Seq with results. But I
> would
> > use conduits and conduit `selectAll`
> >
> http://hackage.haskell.org/package/hdbi-conduit-1.3.0/docs/Data-Conduit-HDBI.html
> .
> > Conduits are realy simple and effective.
> >
> > If you need something like nextresult just use method `fetch` of
> `Statement`
> > until it return Nothing. It is just the same. Or use ResumableSink from
> > conduits (I would).
> >
> >
> >
> > 2013/10/16 Gauthier Segay <gauthier.segay at gmail.com>
> >>
> >> Thanks for the announcement / library, I have started using hdbc (and
> >> it's odbc driver) recently and had two concerns with it so far:
> >>
> >> * ability to use named parameters
> >>
> >> (
> http://stackoverflow.com/questions/19137803/does-database-hdbc-support-named-parameters
> )
> >> * fetching multiple results returned from single statement
> >>
> >> (
> http://stackoverflow.com/questions/19159287/hdbc-and-multiple-resultsets-in-a-single-statement-only-first-resultset-returne
> )
> >> (in .net this is done via
> >>
> >>
> http://msdn.microsoft.com/en-us/library/system.data.idatareader.nextresult.aspx
> )
> >>
> >> Is there any plan to get this supported in HDBI?
> >>
> >> On Tue, Oct 15, 2013 at 7:55 PM, Alexey Uimanov <s9gf4ult at gmail.com>
> >> wrote:
> >> > Hello haskellers!
> >> >
> >> > HDBI is the fork of HDBC but reworked. It supports SQlite3 and
> >> > Postgresql
> >> > for now. It also supports streaming with coduits.
> >> > There is TH deriving mechanism to map database rows to Haskell
> >> > structures
> >> > and back. HDBI is trying to become simple
> >> > but still powerfull and flexible database interface. It must be
> suitable
> >> > to
> >> > become the common RDBMS interface  for higher level interfaces
> >> > like persistent or haskelldb.
> >> >
> >> > The documentation is not very good, while I have no enough time to
> make
> >> > some.
> >> >
> >> > In this version changed typeclass signatures of Connection and
> >> > Statement.
> >> > Now methods `run` and `execute` get any instance of `ToRow` and method
> >> > `fetch` return an instance of `FromRow`.
> >> > Note that [SqlValue] is also an instance of `FromRow` and `ToRow`
> >> > typeclasses so you do not loose the control.
> >> > Methods `fromRow` and `toRow` for [SqlValue] are just `id`. SQlite and
> >> > Postgresql drivers are fixed as well as hdbi-conduit.
> >> >
> >> > There is also new helper functions, like `onei :: Integer ->
> [SqlValue]`
> >> > which helps you to execute queries with one parameter
> >> > or execute many queries consistinf of one parameter.
> >> >
> >> > Prelude Database.HDBI Database.HDBI.SQlite> :set -XScopedTypeVariables
> >> > Prelude Database.HDBI Database.HDBI.SQlite> :set -XOverloadedStrings
> >> > Prelude Database.HDBI Database.HDBI.SQlite> c <- connectSqlite3
> >> > ":memory:"
> >> > Prelude Database.HDBI Database.HDBI.SQlite> runRaw c "create table
> >> > test(val
> >> > integer)"
> >> > Prelude Database.HDBI Database.HDBI.SQlite> withTransaction c $
> runMany
> >> > c
> >> > "insert into test(val) values (?)" $ map one [1..1000]
> >> >
> >> > <interactive>:7:76: Warning:
> >> >     Defaulting the following constraint(s) to type `Integer'
> >> > .........
> >> >
> >> > Prelude Database.HDBI Database.HDBI.SQlite> r :: (Maybe Integer) <-
> >> > runFetchOne c "select sum(val) from test" ()
> >> > Prelude Database.HDBI Database.HDBI.SQlite> r
> >> > Just 500500
> >> >
> >> > Note here that the empty set is used as a parameter of query in
> >> > `runFetchOne`. Empty set is an instance of `FromRow` and `ToRow` and
> >> > return
> >> > an empty list of [SqlValue].
> >> > Use empty list as a parameters is bad idea, because we could
> instantiate
> >> > some another list of things, suppose the [Integer] as `FromRow` and
> >> > `ToRow` instance. So it would lead to ambigous type because [SqlValue]
> >> > is
> >> > also a list instantiating `FromRow` and `ToRow`.
> >> >
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> >> >
> >
> >
> >
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