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112 lines
3.8 KiB
Haskell
112 lines
3.8 KiB
Haskell
module Hasql.Statement
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(
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Statement(..),
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-- * Recipies
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-- ** Insert many
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-- $insertMany
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-- ** IN and NOT IN
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-- $inAndNotIn
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)
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where
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import Hasql.Private.Prelude
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import qualified Hasql.Decoders as Decoders
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import qualified Hasql.Encoders as Encoders
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{-|
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Specification of a strictly single-statement query, which can be parameterized and prepared.
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Consists of the following:
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* SQL template,
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* params encoder,
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* result decoder,
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* a flag, determining whether it should be prepared.
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The SQL template must be formatted according to Postgres' standard,
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with any non-ASCII characters of the template encoded using UTF-8.
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According to the format,
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parameters must be referred to using a positional notation, as in the following:
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@$1@, @$2@, @$3@ and etc.
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Those references must be used in accordance to the order in which the according
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value encoders are specified in 'Encoders.Params'.
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Following is an example of a declaration of a prepared statement with its associated codecs.
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@
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selectSum :: 'Statement' (Int64, Int64) Int64
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selectSum = 'Statement' sql encoder decoder True where
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sql = "select ($1 + $2)"
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encoder =
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('fst' '>$<' Encoders.'Hasql.Encoders.param' (Encoders.'Hasql.Encoders.nonNullable' Encoders.'Hasql.Encoders.int8')) '<>'
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('snd' '>$<' Encoders.'Hasql.Encoders.param' (Encoders.'Hasql.Encoders.nullable' Encoders.'Hasql.Encoders.text'))
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decoder = Decoders.'Hasql.Decoders.singleRow' (Decoders.'Hasql.Decoders.column' (Decoders.'Hasql.Decoders.nonNullable' Decoders.'Hasql.Decoders.int8'))
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@
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The statement above accepts a product of two parameters of type 'Int64'
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and produces a single result of type 'Int64'.
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-}
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data Statement a b =
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Statement ByteString (Encoders.Params a) (Decoders.Result b) Bool
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instance Functor (Statement a) where
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{-# INLINE fmap #-}
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fmap = rmap
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instance Profunctor Statement where
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{-# INLINE dimap #-}
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dimap f1 f2 (Statement template encoder decoder preparable) =
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Statement template (contramap f1 encoder) (fmap f2 decoder) preparable
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{- $insertMany
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It is not currently possible to pass in an array of encodable values
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to use in an insert many statement. Instead, PostgreSQL's
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(9.4 or later) @unnest@ function can be used in an analogous way
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to haskell's `zip` function by passing in multiple arrays of values
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to be zipped into the rows we want to insert:
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@
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insertMultipleLocations :: 'Statement' (Vector (UUID, Double, Double)) ()
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insertMultipleLocations = 'Statement' sql encoder decoder True where
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sql = "insert into location (id, x, y) select * from unnest ($1, $2, $3)"
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encoder =
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contramap Vector.'Data.Vector.unzip3' $
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contrazip3 (vector Encoders.'Encoders.uuid') (vector Encoders.'Encoders.float8') (vector Encoders.'Encoders.float8')
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where
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vector =
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Encoders.'Encoders.param' .
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Encoders.'Encoders.nonNullable' .
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Encoders.'Encoders.array' .
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Encoders.'Encoders.dimension' 'foldl'' .
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Encoders.'Encoders.element' .
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Encoders.'Encoders.nonNullable'
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decoder = Decoders.'Decoders.noResult'
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@
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This approach is much more efficient than executing a single-row Insert
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statement multiple times.
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-}
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{- $inAndNotIn
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There is a common misconception that Postgresql supports array
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as a parameter for the @IN@ operator.
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However Postgres only supports a syntactical list of values with it,
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i.e., you have to specify each option as an individual parameter
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(@something IN ($1, $2, $3)@).
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Clearly it would be much more convenient to provide an array as a single parameter,
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but the @IN@ operator does not support that.
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Fortunately, Postgres does provide such functionality with other operators:
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* Use @something = ANY($1)@ instead of @something IN ($1)@
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* Use @something <> ALL($1)@ instead of @something NOT IN ($1)@
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For details see
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<https://www.postgresql.org/docs/9.6/static/functions-comparisons.html#AEN20944 the Postgresql docs>.
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-}
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