| Copyright | (c) 2011 MailRank, Inc. (c) 2011-2012 Leon P Smith |
|---|---|
| License | BSD3 |
| Maintainer | Leon P Smith <leon@melding-monads.com> |
| Stability | experimental |
| Safe Haskell | None |
| Language | Haskell98 |
Database.PostgreSQL.Simple.Types
Description
Basic types.
- data Null = Null
- data Default = Default
- newtype Only a = Only {
- fromOnly :: a
- newtype In a = In a
- newtype Binary a = Binary {
- fromBinary :: a
- newtype Identifier = Identifier {}
- data QualifiedIdentifier = QualifiedIdentifier (Maybe Text) Text
- newtype Query = Query {}
- newtype Oid :: * = Oid CUInt
- data h :. t = h :. t
- newtype Savepoint = Savepoint Query
- newtype PGArray a = PGArray {
- fromPGArray :: [a]
- data Values a = Values [QualifiedIdentifier] [a]
Documentation
A placeholder for the SQL NULL value.
Constructors
| Null |
A placeholder for the PostgreSQL DEFAULT value.
Constructors
| Default |
A single-value "collection".
This is useful if you need to supply a single parameter to a SQL query, or extract a single column from a SQL result.
Parameter example:
query c "select x from scores where x > ?" (Only (42::Int))Result example:
xs <- query_ c "select id from users"
forM_ xs $ \(Only id) -> {- ... -}Wrap a list of values for use in an IN clause. Replaces a
single "?" character with a parenthesized list of rendered
values.
Example:
query c "select * from whatever where id in ?" (Only (In [3,4,5]))
Note that In [] expands to (null), which works as expected in
the query above, but evaluates to the logical null value on every
row instead of TRUE. This means that changing the query above
to ... id NOT in ? and supplying the empty list as the parameter
returns zero rows, instead of all of them as one would expect.
Since postgresql doesn't seem to provide a syntax for actually specifying an empty list, which could solve this completely, there are two workarounds particularly worth mentioning, namely:
Use postgresql-simple's
Valuestype instead, which can handle the empty case correctly. Note however that while specifying the postgresql type"int4"is mandatory in the empty case, specifying the haskell typeValues (Only Int)would not normally be needed in realistic use cases.query c "select * from whatever where id not in ?" (Only (Values ["int4"] [] :: Values (Only Int)))Use sql's
COALESCEoperator to turn a logicalnullinto the correct boolean. Note however that the correct boolean depends on the use case:query c "select * from whatever where coalesce(id NOT in ?, TRUE)" (Only (In [] :: In [Int]))query c "select * from whatever where coalesce(id IN ?, FALSE)" (Only (In [] :: In [Int]))Note that at as of PostgreSQL 9.4, the query planner cannot see inside the
COALESCEoperator, so if you have an index onidthen you probably don't want to write the last example withCOALESCE, which would result in a table scan. There are further caveats ifidcan be null or you want null treated sensibly as a component ofINorNOT IN.
Constructors
| In a |
Wrap binary data for use as a bytea value.
Constructors
| Binary | |
Fields
| |
Instances
| Functor Binary Source # | |
| Eq a => Eq (Binary a) Source # | |
| Ord a => Ord (Binary a) Source # | |
| Read a => Read (Binary a) Source # | |
| Show a => Show (Binary a) Source # | |
| FromField (Binary ByteString) Source # | bytea |
| FromField (Binary ByteString) Source # | bytea |
| ToField (Binary ByteString) Source # | |
| ToField (Binary ByteString) Source # | |
newtype Identifier Source #
Wrap text for use as sql identifier, i.e. a table or column name.
Constructors
| Identifier | |
Fields | |
data QualifiedIdentifier Source #
Wrap text for use as (maybe) qualified identifier, i.e. a table with schema, or column with table.
Constructors
| QualifiedIdentifier (Maybe Text) Text |
Instances
| Eq QualifiedIdentifier Source # | |
| Ord QualifiedIdentifier Source # | |
| Read QualifiedIdentifier Source # | |
| Show QualifiedIdentifier Source # | |
| IsString QualifiedIdentifier Source # |
|
| Hashable QualifiedIdentifier Source # | |
| ToField QualifiedIdentifier Source # | |
A query string. This type is intended to make it difficult to construct a SQL query by concatenating string fragments, as that is an extremely common way to accidentally introduce SQL injection vulnerabilities into an application.
This type is an instance of IsString, so the easiest way to
construct a query is to enable the OverloadedStrings language
extension and then simply write the query in double quotes.
{-# LANGUAGE OverloadedStrings #-}
import Database.PostgreSQL.Simple
q :: Query
q = "select ?"The underlying type is a ByteString, and literal Haskell strings
that contain Unicode characters will be correctly transformed to
UTF-8.
Constructors
| Query | |
Fields | |
A composite type to parse your custom data structures without having to define dummy newtype wrappers every time.
instance FromRow MyData where ...
instance FromRow MyData2 where ...
then I can do the following for free:
res <- query' c "..."
forM res $ \(MyData{..} :. MyData2{..}) -> do
....
Constructors
| h :. t infixr 3 |
Wrap a list for use as a PostgreSQL array.
Constructors
| PGArray | |
Fields
| |
Instances
| Functor PGArray Source # | |
| Eq a => Eq (PGArray a) Source # | |
| Ord a => Ord (PGArray a) Source # | |
| Read a => Read (PGArray a) Source # | |
| Show a => Show (PGArray a) Source # | |
| (FromField a, Typeable * a) => FromField (PGArray a) Source # | any postgresql array whose elements are compatible with type |
| ToField a => ToField (PGArray a) Source # | |
Represents a VALUES table literal, usable as an alternative to
executeMany and
returning. The main advantage is that
you can parametrize more than just a single VALUES expression.
For example, here's a query to insert a thing into one table
and some attributes of that thing into another, returning the
new id generated by the database:
query c [sql|
WITH new_thing AS (
INSERT INTO thing (name) VALUES (?) RETURNING id
), new_attributes AS (
INSERT INTO thing_attributes
SELECT new_thing.id, attrs.*
FROM new_thing JOIN ? attrs ON TRUE
) SELECT * FROM new_thing
|] ("foo", Values [ "int4", "text" ]
[ ( 1 , "hello" )
, ( 2 , "world" ) ])(Note this example uses writable common table expressions, which were added in PostgreSQL 9.1)
The second parameter gets expanded into the following SQL syntax:
(VALUES (1::"int4",'hello'::"text"),(2,'world'))
When the list of attributes is empty, the second parameter expands to:
(VALUES (null::"int4",null::"text") LIMIT 0)
By contrast, executeMany and returning don't issue the query
in the empty case, and simply return 0 and [] respectively.
This behavior is usually correct given their intended use cases,
but would certainly be wrong in the example above.
The first argument is a list of postgresql type names. Because this
is turned into a properly quoted identifier, the type name is case
sensitive and must be as it appears in the pg_type table. Thus,
you must write timestamptz instead of timestamp with time zone,
int4 instead of integer or serial, _int8 instead of bigint[],
etcetera.
You may omit the type names, however, if you do so the list
of values must be non-empty, and postgresql must be able to infer
the types of the columns from the surrounding context. If the first
condition is not met, postgresql-simple will throw an exception
without issuing the query. In the second case, the postgres server
will return an error which will be turned into a SqlError exception.
See https://www.postgresql.org/docs/9.5/static/sql-values.html for more information.
Constructors
| Values [QualifiedIdentifier] [a] |