[Haskell-cafe] Announce: HDBI-1.3 and friends
Vagif Verdi
vagif.verdi at gmail.com
Wed Oct 16 15:22:51 UTC 2013
Over my more than 20 years career working with many different databases and
languages you are the first person i meet who requests such a feature.
Premature optimization is harmful.
You are looking for a solution to a problem that practically never
manifests itself.
Especially in a server side application that are usually hosted close to
sql server and on a very fat bandwidth.
In terms of the performance much more important question is connection
pooling.
You are basically asking a single programmer who just started a quite big
project to spend his time on a feature practically no one is interested
with while he has hundreds of much more pressing issues to implement.
I'd say at this point your only course of action is to implement it
yourself, if it is really that important to you.
On Wednesday, October 16, 2013 6:42:56 AM UTC-7, Gauthier Segay wrote:
>
> no the use case is to issue a bunch of disparate select statements,
> i.e. retrieving data from several tables in a single roundtrip.
>
> select 1,2,3
>
> select 'a', 'b'
>
> In this case, I'd like to call the API in this manner (or something
> better):
>
> -- statement is prepared
> rows1 <- fetchAllRows statement
> _ <- nextResults statement
> rows2 <- fetchAllRows statement
> -- continue
>
> right now, with hdbc-odbc (nextResults doesn't exist though so I don't
> call it) rows2 will be an empty list, I'd expect it to be[(SqlText
> "a"), (SqlText "b")]
>
> I assume in the case of odbc, the related C function is SQLMoreResults
>
> http://msdn.microsoft.com/en-us/library/ms714673%28v=vs.85%29.aspx
>
>
> On Wed, Oct 16, 2013 at 3:16 PM, Alexey Uimanov <s9gf... at gmail.com<javascript:>>
> wrote:
> > 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 <gauthie... at gmail.com <javascript:>>
> >>
> >> > 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 <s9gf... at gmail.com<javascript:>>
>
> >> 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 <gauthie... at gmail.com <javascript:>>
> >> >>
> >> >> 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 <s9gf... at gmail.com<javascript:>>
>
> >> >> 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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