{-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE NamedFieldPuns #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE PatternGuards #-} {-# LANGUAGE RecordWildCards #-} {-# LANGUAGE ViewPatterns #-} ------------------------------------------------------------------------------ -- | -- Module: Database.PostgreSQL.Simple -- Copyright: (c) 2011 MailRank, Inc. -- (c) 2011-2012 Leon P Smith -- License: BSD3 -- Maintainer: Leon P Smith <leon@melding-monads.com> -- Stability: experimental -- -- A mid-level client library for the PostgreSQL database, aimed at ease of -- use and high performance. -- ------------------------------------------------------------------------------ module Database.PostgreSQL.Simple ( -- * Writing queries -- $use -- ** The Query type -- $querytype -- ** Parameter substitution -- $subst -- *** Type inference -- $inference -- ** Substituting a single parameter -- $only_param -- ** Representing a list of values -- $in -- ** Modifying multiple rows at once -- $many -- ** @RETURNING@: modifications that returns results -- $returning -- * Extracting results -- $result -- ** Handling null values -- $null -- ** Type conversions -- $types -- * Types Connection , Query , ToRow , FromRow , In(..) , Binary(..) , Only(..) , (:.)(..) -- ** Exceptions , SqlError(..) , PQ.ExecStatus(..) , FormatError(..) , QueryError(..) , ResultError(..) -- * Connection management , Base.connectPostgreSQL , Base.close , Base.connect , Base.ConnectInfo(..) , Base.defaultConnectInfo , Base.postgreSQLConnectionString -- * Queries that return results , query , query_ -- ** Queries taking parser as argument , queryWith , queryWith_ -- * Queries that stream results , FoldOptions(..) , FetchQuantity(..) , defaultFoldOptions , fold , foldWithOptions , fold_ , foldWithOptions_ , forEach , forEach_ , returning -- * Statements that do not return results , execute , execute_ , executeMany -- , Base.insertID -- * Transaction handling , withTransaction , withSavepoint -- , Base.autocommit , begin , commit , rollback -- * Helper functions , formatMany , formatQuery ) where import Blaze.ByteString.Builder ( Builder, fromByteString, toByteString ) import Blaze.ByteString.Builder.Char8 (fromChar) import Blaze.Text ( integral ) import Control.Applicative ((<$>)) import Control.Exception as E import Control.Monad (foldM) import Data.ByteString (ByteString) import Data.Int (Int64) import Data.List (intersperse) import Data.Monoid (mconcat) import Database.PostgreSQL.Simple.Compat ( (<>) ) import Database.PostgreSQL.Simple.FromField (ResultError(..)) import Database.PostgreSQL.Simple.FromRow (FromRow(..)) import Database.PostgreSQL.Simple.Ok import Database.PostgreSQL.Simple.ToField (Action(..)) import Database.PostgreSQL.Simple.ToRow (ToRow(..)) import Database.PostgreSQL.Simple.Types ( Binary(..), In(..), Only(..), Query(..), (:.)(..) ) import Database.PostgreSQL.Simple.Internal as Base import Database.PostgreSQL.Simple.Transaction import Database.PostgreSQL.Simple.TypeInfo import qualified Database.PostgreSQL.LibPQ as PQ import qualified Data.ByteString.Char8 as B import Control.Monad.Trans.Reader import Control.Monad.Trans.State.Strict -- | Format a query string. -- -- This function is exposed to help with debugging and logging. Do not -- use it to prepare queries for execution. -- -- String parameters are escaped according to the character set in use -- on the 'Connection'. -- -- Throws 'FormatError' if the query string could not be formatted -- correctly. formatQuery :: ToRow q => Connection -> Query -> q -> IO ByteString formatQuery conn q@(Query template) qs | null xs && '?' `B.notElem` template = return template | otherwise = toByteString <$> buildQuery conn q template xs where xs = toRow qs -- | Format a query string with a variable number of rows. -- -- This function is exposed to help with debugging and logging. Do not -- use it to prepare queries for execution. -- -- The query string must contain exactly one substitution group, -- identified by the SQL keyword \"@VALUES@\" (case insensitive) -- followed by an \"@(@\" character, a series of one or more \"@?@\" -- characters separated by commas, and a \"@)@\" character. White -- space in a substitution group is permitted. -- -- Throws 'FormatError' if the query string could not be formatted -- correctly. formatMany :: (ToRow q) => Connection -> Query -> [q] -> IO ByteString formatMany _ q [] = fmtError "no rows supplied" q [] formatMany conn q@(Query template) qs = do case parseTemplate template of Just (before, qbits, after) -> do bs <- mapM (buildQuery conn q qbits . toRow) qs return . toByteString . mconcat $ fromByteString before : intersperse (fromChar ',') bs ++ [fromByteString after] Nothing -> fmtError "syntax error in multi-row template" q [] -- Split the input string into three pieces, @before@, @qbits@, and @after@, -- following this grammar: -- -- start: ^ before qbits after $ -- before: ([^?]* [^?\w])? 'VALUES' \s* -- qbits: '(' \s* '?' \s* (',' \s* '?' \s*)* ')' -- after: [^?]* -- -- \s: [ \t\n\r\f] -- \w: [A-Z] | [a-z] | [\x80-\xFF] | '_' | '$' | [0-9] -- -- This would be much more concise with some sort of regex engine. -- 'formatMany' used to use pcre-light instead of this hand-written parser, -- but pcre is a hassle to install on Windows. parseTemplate :: ByteString -> Maybe (ByteString, ByteString, ByteString) parseTemplate template = -- Convert input string to uppercase, to facilitate searching. search $ B.map toUpper_ascii template where -- Search for the next occurrence of "VALUES" search bs = case B.breakSubstring "VALUES" bs of (x, y) -- If "VALUES" is not present in the string, or any '?' characters -- were encountered prior to it, fail. | B.null y || ('?' `B.elem` x) -> Nothing -- If "VALUES" is preceded by an identifier character (a.k.a. \w), -- try the next occurrence. | not (B.null x) && isIdent (B.last x) -> search $ B.drop 6 y -- Otherwise, we have a legitimate "VALUES" token. | otherwise -> parseQueryBits $ skipSpace $ B.drop 6 y -- Parse '(' \s* '?' \s* . If this doesn't match -- (and we don't consume a '?'), look for another "VALUES". -- -- qb points to the open paren (if present), meaning it points to the -- beginning of the "qbits" production described above. This is why we -- pass it down to finishQueryBits. parseQueryBits qb | Just ('(', skipSpace -> bs1) <- B.uncons qb , Just ('?', skipSpace -> bs2) <- B.uncons bs1 = finishQueryBits qb bs2 | otherwise = search qb -- Parse (',' \s* '?' \s*)* ')' [^?]* . -- -- Since we've already consumed at least one '?', there's no turning back. -- The parse has to succeed here, or the whole thing fails -- (because we don't allow '?' to appear outside of the VALUES list). finishQueryBits qb bs0 | Just (')', bs1) <- B.uncons bs0 = if '?' `B.elem` bs1 then Nothing else Just $ slice3 template qb bs1 | Just (',', skipSpace -> bs1) <- B.uncons bs0 , Just ('?', skipSpace -> bs2) <- B.uncons bs1 = finishQueryBits qb bs2 | otherwise = Nothing -- Slice a string into three pieces, given the start offset of the second -- and third pieces. Each "offset" is actually a tail of the uppercase -- version of the template string. Its length is used to infer the offset. -- -- It is important to note that we only slice the original template. -- We don't want our all-caps trick messing up the actual query string. slice3 source p1 p2 = (s1, s2, source'') where (s1, source') = B.splitAt (B.length source - B.length p1) source (s2, source'') = B.splitAt (B.length p1 - B.length p2) source' toUpper_ascii c | c >= 'a' && c <= 'z' = toEnum (fromEnum c - 32) | otherwise = c -- Based on the definition of {ident_cont} in src/backend/parser/scan.l -- in the PostgreSQL source. No need to check [a-z], since we converted -- the whole string to uppercase. isIdent c = (c >= '0' && c <= '9') || (c >= 'A' && c <= 'Z') || (c >= '\x80' && c <= '\xFF') || c == '_' || c == '$' -- Based on {space} in scan.l isSpace_ascii c = (c == ' ') || (c >= '\t' && c <= '\r') skipSpace = B.dropWhile isSpace_ascii buildQuery :: Connection -> Query -> ByteString -> [Action] -> IO Builder buildQuery conn q template xs = zipParams (split template) <$> mapM (buildAction conn q xs) xs where split s = let (h,t) = B.break (=='?') s in fromByteString h : if B.null t then [] else split (B.tail t) zipParams (t:ts) (p:ps) = t <> p <> zipParams ts ps zipParams [t] [] = t zipParams _ _ = fmtError (show (B.count '?' template) ++ " '?' characters, but " ++ show (length xs) ++ " parameters") q xs -- | Execute an @INSERT@, @UPDATE@, or other SQL query that is not -- expected to return results. -- -- Returns the number of rows affected. -- -- Throws 'FormatError' if the query could not be formatted correctly, or -- a 'SqlError' exception if the backend returns an error. execute :: (ToRow q) => Connection -> Query -> q -> IO Int64 execute conn template qs = do result <- exec conn =<< formatQuery conn template qs finishExecute conn template result -- | Execute a multi-row @INSERT@, @UPDATE@, or other SQL query that is not -- expected to return results. -- -- Returns the number of rows affected. If the list of parameters is empty, -- this function will simply return 0 without issuing the query to the backend. -- If this is not desired, consider using the 'Values' constructor instead. -- -- Throws 'FormatError' if the query could not be formatted correctly, or -- a 'SqlError' exception if the backend returns an error. -- -- For example, here's a command that inserts two rows into a table -- with two columns: -- -- @ -- executeMany c [sql| -- INSERT INTO sometable VALUES (?,?) -- |] [(1, \"hello\"),(2, \"world\")] -- @ -- -- Here's an canonical example of a multi-row update command: -- -- @ -- executeMany c [sql| -- UPDATE sometable -- SET sometable.y = upd.y -- FROM (VALUES (?,?)) as upd(x,y) -- WHERE sometable.x = upd.x -- |] [(1, \"hello\"),(2, \"world\") -- @ executeMany :: (ToRow q) => Connection -> Query -> [q] -> IO Int64 executeMany _ _ [] = return 0 executeMany conn q qs = do result <- exec conn =<< formatMany conn q qs finishExecute conn q result -- | Execute @INSERT ... RETURNING@, @UPDATE ... RETURNING@, or other SQL -- query that accepts multi-row input and is expected to return results. -- Note that it is possible to write -- @'query' conn "INSERT ... RETURNING ..." ...@ -- in cases where you are only inserting a single row, and do not need -- functionality analogous to 'executeMany'. -- -- If the list of parameters is empty, this function will simply return @[]@ -- without issuing the query to the backend. If this is not desired, -- consider using the 'Values' constructor instead. -- -- Throws 'FormatError' if the query could not be formatted correctly. returning :: (ToRow q, FromRow r) => Connection -> Query -> [q] -> IO [r] returning _ _ [] = return [] returning conn q qs = do result <- exec conn =<< formatMany conn q qs finishQuery conn q result -- | Perform a @SELECT@ or other SQL query that is expected to return -- results. All results are retrieved and converted before this -- function returns. -- -- When processing large results, this function will consume a lot of -- client-side memory. Consider using 'fold' instead. -- -- Exceptions that may be thrown: -- -- * 'FormatError': the query string could not be formatted correctly. -- -- * 'QueryError': the result contains no columns (i.e. you should be -- using 'execute' instead of 'query'). -- -- * 'ResultError': result conversion failed. -- -- * 'SqlError': the postgresql backend returned an error, e.g. -- a syntax or type error, or an incorrect table or column name. query :: (ToRow q, FromRow r) => Connection -> Query -> q -> IO [r] query = queryWith fromRow -- | A version of 'query' that does not perform query substitution. query_ :: (FromRow r) => Connection -> Query -> IO [r] query_ = queryWith_ fromRow -- | A version of 'query' taking parser as argument queryWith :: ToRow q => RowParser r -> Connection -> Query -> q -> IO [r] queryWith parser conn template qs = do result <- exec conn =<< formatQuery conn template qs finishQueryWith parser conn template result -- | A version of 'query_' taking parser as argument queryWith_ :: RowParser r -> Connection -> Query -> IO [r] queryWith_ parser conn q@(Query que) = do result <- exec conn que finishQueryWith parser conn q result -- | Perform a @SELECT@ or other SQL query that is expected to return -- results. Results are streamed incrementally from the server, and -- consumed via a left fold. -- -- When dealing with small results, it may be simpler (and perhaps -- faster) to use 'query' instead. -- -- This fold is /not/ strict. The stream consumer is responsible for -- forcing the evaluation of its result to avoid space leaks. -- -- This is implemented using a database cursor. As such, this requires -- a transaction. This function will detect whether or not there is a -- transaction in progress, and will create a 'ReadCommitted' 'ReadOnly' -- transaction if needed. The cursor is given a unique temporary name, -- so the consumer may itself call fold. -- -- Exceptions that may be thrown: -- -- * 'FormatError': the query string could not be formatted correctly. -- -- * 'QueryError': the result contains no columns (i.e. you should be -- using 'execute' instead of 'query'). -- -- * 'ResultError': result conversion failed. -- -- * 'SqlError': the postgresql backend returned an error, e.g. -- a syntax or type error, or an incorrect table or column name. fold :: ( FromRow row, ToRow params ) => Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a fold = foldWithOptions defaultFoldOptions -- | Number of rows to fetch at a time. 'Automatic' currently defaults -- to 256 rows, although it might be nice to make this more intelligent -- based on e.g. the average size of the rows. data FetchQuantity = Automatic | Fixed !Int data FoldOptions = FoldOptions { fetchQuantity :: !FetchQuantity, transactionMode :: !TransactionMode } -- | defaults to 'Automatic', and 'TransactionMode' 'ReadCommitted' 'ReadOnly' defaultFoldOptions :: FoldOptions defaultFoldOptions = FoldOptions { fetchQuantity = Automatic, transactionMode = TransactionMode ReadCommitted ReadOnly } -- | The same as 'fold', but this provides a bit more control over -- lower-level details. Currently, the number of rows fetched per -- round-trip to the server and the transaction mode may be adjusted -- accordingly. If the connection is already in a transaction, -- then the existing transaction is used and thus the 'transactionMode' -- option is ignored. foldWithOptions :: ( FromRow row, ToRow params ) => FoldOptions -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a foldWithOptions opts conn template qs a f = do q <- formatQuery conn template qs doFold opts conn template (Query q) a f -- | A version of 'fold' that does not perform query substitution. fold_ :: (FromRow r) => Connection -> Query -- ^ Query. -> a -- ^ Initial state for result consumer. -> (a -> r -> IO a) -- ^ Result consumer. -> IO a fold_ = foldWithOptions_ defaultFoldOptions foldWithOptions_ :: (FromRow r) => FoldOptions -> Connection -> Query -- ^ Query. -> a -- ^ Initial state for result consumer. -> (a -> r -> IO a) -- ^ Result consumer. -> IO a foldWithOptions_ opts conn query a f = doFold opts conn query query a f doFold :: ( FromRow row ) => FoldOptions -> Connection -> Query -> Query -> a -> (a -> row -> IO a) -> IO a doFold FoldOptions{..} conn _template q a0 f = do stat <- withConnection conn PQ.transactionStatus case stat of PQ.TransIdle -> withTransactionMode transactionMode conn go PQ.TransInTrans -> go PQ.TransActive -> fail "foldWithOpts FIXME: PQ.TransActive" -- This _shouldn't_ occur in the current incarnation of -- the library, as we aren't using libpq asynchronously. -- However, it could occur in future incarnations of -- this library or if client code uses the Internal module -- to use raw libpq commands on postgresql-simple connections. PQ.TransInError -> fail "foldWithOpts FIXME: PQ.TransInError" -- This should be turned into a better error message. -- It is probably a bad idea to automatically roll -- back the transaction and start another. PQ.TransUnknown -> fail "foldWithOpts FIXME: PQ.TransUnknown" -- Not sure what this means. where declare = do name <- newTempName conn _ <- execute_ conn $ mconcat [ "DECLARE ", name, " NO SCROLL CURSOR FOR ", q ] return name fetch (Query name) = query_ conn $ Query (toByteString (fromByteString "FETCH FORWARD " <> integral chunkSize <> fromByteString " FROM " <> fromByteString name )) close name = (execute_ conn ("CLOSE " <> name) >> return ()) `E.catch` \ex -> -- Don't throw exception if CLOSE failed because the transaction is -- aborted. Otherwise, it will throw away the original error. if isFailedTransactionError ex then return () else throwIO ex go = bracket declare close $ \name -> let loop a = do rs <- fetch name if null rs then return a else foldM f a rs >>= loop in loop a0 -- FIXME: choose the Automatic chunkSize more intelligently -- One possibility is to use the type of the results, although this -- still isn't a perfect solution, given that common types (e.g. text) -- are of highly variable size. -- A refinement of this technique is to pick this number adaptively -- as results are read in from the database. chunkSize = case fetchQuantity of Automatic -> 256 Fixed n -> n -- | A version of 'fold' that does not transform a state value. forEach :: (ToRow q, FromRow r) => Connection -> Query -- ^ Query template. -> q -- ^ Query parameters. -> (r -> IO ()) -- ^ Result consumer. -> IO () forEach conn template qs = fold conn template qs () . const {-# INLINE forEach #-} -- | A version of 'forEach' that does not perform query substitution. forEach_ :: (FromRow r) => Connection -> Query -- ^ Query template. -> (r -> IO ()) -- ^ Result consumer. -> IO () forEach_ conn template = fold_ conn template () . const {-# INLINE forEach_ #-} forM' :: (Ord n, Num n) => n -> n -> (n -> IO a) -> IO [a] forM' lo hi m = loop hi [] where loop !n !as | n < lo = return as | otherwise = do a <- m n loop (n-1) (a:as) finishQuery :: FromRow r => Connection -> Query -> PQ.Result -> IO [r] finishQuery = finishQueryWith fromRow finishQueryWith :: RowParser r -> Connection -> Query -> PQ.Result -> IO [r] finishQueryWith parser conn q result = do status <- PQ.resultStatus result case status of PQ.EmptyQuery -> throwIO $ QueryError "query: Empty query" q PQ.CommandOk -> do throwIO $ QueryError "query resulted in a command response" q PQ.TuplesOk -> do let unCol (PQ.Col x) = fromIntegral x :: Int nrows <- PQ.ntuples result ncols <- PQ.nfields result forM' 0 (nrows-1) $ \row -> do let rw = Row row result okvc <- runConversion (runStateT (runReaderT (unRP parser) rw) 0) conn case okvc of Ok (val,col) | col == ncols -> return val | otherwise -> do vals <- forM' 0 (ncols-1) $ \c -> do tinfo <- getTypeInfo conn =<< PQ.ftype result c v <- PQ.getvalue result row c return ( tinfo , fmap ellipsis v ) throw (ConversionFailed (show (unCol ncols) ++ " values: " ++ show vals) Nothing "" (show (unCol col) ++ " slots in target type") "mismatch between number of columns to \ \convert and number in target type") Errors [] -> throwIO $ ConversionFailed "" Nothing "" "" "unknown error" Errors [x] -> throwIO x Errors xs -> throwIO $ ManyErrors xs PQ.CopyOut -> throwIO $ QueryError "query: COPY TO is not supported" q PQ.CopyIn -> throwIO $ QueryError "query: COPY FROM is not supported" q PQ.BadResponse -> throwResultError "query" result status PQ.NonfatalError -> throwResultError "query" result status PQ.FatalError -> throwResultError "query" result status ellipsis :: ByteString -> ByteString ellipsis bs | B.length bs > 15 = B.take 10 bs `B.append` "[...]" | otherwise = bs -- $use -- -- SQL-based applications are somewhat notorious for their -- susceptibility to attacks through the injection of maliciously -- crafted data. The primary reason for widespread vulnerability to -- SQL injections is that many applications are sloppy in handling -- user data when constructing SQL queries. -- -- This library provides a 'Query' type and a parameter substitution -- facility to address both ease of use and security. -- $querytype -- -- A 'Query' is a @newtype@-wrapped 'ByteString'. It intentionally -- exposes a tiny API that is not compatible with the 'ByteString' -- API; this makes it difficult to construct queries from fragments of -- strings. The 'query' and 'execute' functions require queries to be -- of type 'Query'. -- -- To most easily construct a query, enable GHC's @OverloadedStrings@ -- language extension and write your query as a normal literal string. -- -- > {-# LANGUAGE OverloadedStrings #-} -- > -- > import Database.PostgreSQL.Simple -- > -- > hello :: IO Int -- > hello = do -- > conn <- connectPostgreSQL "" -- > [Only i] <- query_ conn "select 2 + 2" -- > return i -- -- A 'Query' value does not represent the actual query that will be -- executed, but is a template for constructing the final query. -- $subst -- -- Since applications need to be able to construct queries with -- parameters that change, this library provides a query substitution -- capability. -- -- The 'Query' template accepted by 'query' and 'execute' can contain -- any number of \"@?@\" characters. Both 'query' and 'execute' -- accept a third argument, typically a tuple. When constructing the -- real query to execute, these functions replace the first \"@?@\" in -- the template with the first element of the tuple, the second -- \"@?@\" with the second element, and so on. If necessary, each -- tuple element will be quoted and escaped prior to substitution; -- this defeats the single most common injection vector for malicious -- data. -- -- For example, given the following 'Query' template: -- -- > select * from user where first_name = ? and age > ? -- -- And a tuple of this form: -- -- > ("Boris" :: String, 37 :: Int) -- -- The query to be executed will look like this after substitution: -- -- > select * from user where first_name = 'Boris' and age > 37 -- -- If there is a mismatch between the number of \"@?@\" characters in -- your template and the number of elements in your tuple, a -- 'FormatError' will be thrown. -- -- Note that the substitution functions do not attempt to parse or -- validate your query. It's up to you to write syntactically valid -- SQL, and to ensure that each \"@?@\" in your query template is -- matched with the right tuple element. -- $inference -- -- Automated type inference means that you will often be able to avoid -- supplying explicit type signatures for the elements of a tuple. -- However, sometimes the compiler will not be able to infer your -- types. Consider a case where you write a numeric literal in a -- parameter tuple: -- -- > query conn "select ? + ?" (40,2) -- -- The above query will be rejected by the compiler, because it does -- not know the specific numeric types of the literals @40@ and @2@. -- This is easily fixed: -- -- > query conn "select ? + ?" (40 :: Double, 2 :: Double) -- -- The same kind of problem can arise with string literals if you have -- the @OverloadedStrings@ language extension enabled. Again, just -- use an explicit type signature if this happens. -- -- Finally, remember that the compiler must be able to infer the type -- of a query's /results/ as well as its parameters. We might like -- the following example to work: -- -- > print =<< query_ conn "select 2 + 2" -- -- Unfortunately, while a quick glance tells us that the result type -- should be a single row containing a single numeric column, the -- compiler has no way to infer what the types are. We can easily fix -- this by providing an explicit type annotation: -- -- > xs <- query_ conn "select 2 + 2" -- > print (xs :: [Only Int]) -- $only_param -- -- Haskell lacks a single-element tuple type, so if you have just one -- value you want substituted into a query or a single-column result, -- what should you do? -- -- The obvious approach would appear to be something like this: -- -- > instance (ToField a) => ToRow a where -- > ... -- -- Unfortunately, this wreaks havoc with type inference, so we take a -- different tack. To represent a single value @val@ as a parameter, write -- a singleton list @[val]@, use 'Just' @val@, or use 'Only' @val@. -- -- Here's an example using a singleton list: -- -- > execute conn "insert into users (first_name) values (?)" -- > ["Nuala"] -- -- A row of /n/ query results is represented using an /n/-tuple, so -- you should use 'Only' to represent a single-column result. -- $in -- -- Suppose you want to write a query using an @IN@ clause: -- -- > select * from users where first_name in ('Anna', 'Boris', 'Carla') -- -- In such cases, it's common for both the elements and length of the -- list after the @IN@ keyword to vary from query to query. -- -- To address this case, use the 'In' type wrapper, and use a single -- \"@?@\" character to represent the list. Omit the parentheses -- around the list; these will be added for you. -- -- Here's an example: -- -- > query conn "select * from users where first_name in ?" $ -- > Only $ In ["Anna", "Boris", "Carla"] -- -- If your 'In'-wrapped list is empty, the string @\"(null)\"@ will be -- substituted instead, to ensure that your clause remains -- syntactically valid. -- $many -- -- If you know that you have many rows of data to insert into a table, -- it is much more efficient to perform all the insertions in a single -- multi-row @INSERT@ statement than individually. -- -- The 'executeMany' function is intended specifically for helping -- with multi-row @INSERT@ and @UPDATE@ statements. Its rules for -- query substitution are different than those for 'execute'. -- -- What 'executeMany' searches for in your 'Query' template is a -- single substring of the form: -- -- > values (?,?,?) -- -- The rules are as follows: -- -- * The keyword @VALUES@ is matched case insensitively. -- -- * There must be no other \"@?@\" characters anywhere in your -- template. -- -- * There must be one or more \"@?@\" in the parentheses. -- -- * Extra white space is fine. -- -- The last argument to 'executeMany' is a list of parameter -- tuples. These will be substituted into the query where the @(?,?)@ -- string appears, in a form suitable for use in a multi-row @INSERT@ -- or @UPDATE@. -- -- Here is an example: -- -- > executeMany conn -- > "insert into users (first_name,last_name) values (?,?)" -- > [("Boris","Karloff"),("Ed","Wood")] -- -- The query that will be executed here will look like this -- (reformatted for tidiness): -- -- > insert into users (first_name,last_name) values -- > ('Boris','Karloff'),('Ed','Wood') -- $returning -- -- PostgreSQL supports returning values from data manipulation statements -- such as @INSERT@ and @UPDATE@. You can use these statements by -- using 'query' instead of 'execute'. For multi-tuple inserts, -- use 'returning' instead of 'executeMany'. -- -- For example, were there an auto-incrementing @id@ column and -- timestamp column @t@ that defaulted to the present time for the -- @sales@ table, then the following query would insert two new -- sales records and also return their new @id@s and timestamps. -- -- > let q = "insert into sales (amount, label) values (?,?) returning id, t" -- > xs :: [(Int, UTCTime)] <- query conn q (15,"Sawdust") -- > ys :: [(Int, UTCTime)] <- returning conn q [(20,"Chips"),(300,"Wood")] -- $result -- -- The 'query' and 'query_' functions return a list of values in the -- 'FromRow' typeclass. This class performs automatic extraction -- and type conversion of rows from a query result. -- -- Here is a simple example of how to extract results: -- -- > import qualified Data.Text as Text -- > -- > xs <- query_ conn "select name,age from users" -- > forM_ xs $ \(name,age) -> -- > putStrLn $ Text.unpack name ++ " is " ++ show (age :: Int) -- -- Notice two important details about this code: -- -- * The number of columns we ask for in the query template must -- exactly match the number of elements we specify in a row of the -- result tuple. If they do not match, a 'ResultError' exception -- will be thrown. -- -- * Sometimes, the compiler needs our help in specifying types. It -- can infer that @name@ must be a 'Text', due to our use of the -- @unpack@ function. However, we have to tell it the type of @age@, -- as it has no other information to determine the exact type. -- $null -- -- The type of a result tuple will look something like this: -- -- > (Text, Int, Int) -- -- Although SQL can accommodate @NULL@ as a value for any of these -- types, Haskell cannot. If your result contains columns that may be -- @NULL@, be sure that you use 'Maybe' in those positions of your -- tuple. -- -- > (Text, Maybe Int, Int) -- -- If 'query' encounters a @NULL@ in a row where the corresponding -- Haskell type is not 'Maybe', it will throw a 'ResultError' -- exception. -- $only_result -- -- To specify that a query returns a single-column result, use the -- 'Only' type. -- -- > xs <- query_ conn "select id from users" -- > forM_ xs $ \(Only dbid) -> {- ... -} -- $types -- -- Conversion of SQL values to Haskell values is somewhat -- permissive. Here are the rules. -- -- * For numeric types, any Haskell type that can accurately represent -- all values of the given PostgreSQL type is considered \"compatible\". -- For instance, you can always extract a PostgreSQL 16-bit @SMALLINT@ -- column to a Haskell 'Int'. The Haskell 'Float' type can accurately -- represent a @SMALLINT@, so it is considered compatble with those types. -- -- * A numeric compatibility check is based only on the type of a -- column, /not/ on its values. For instance, a PostgreSQL 64-bit -- @BIGINT@ column will be considered incompatible with a Haskell -- 'Int16', even if it contains the value @1@. -- -- * If a numeric incompatibility is found, 'query' will throw a -- 'ResultError'. -- -- * The 'String' and 'Text' types are assumed to be encoded as -- UTF-8. If you use some other encoding, decoding may fail or give -- wrong results. In such cases, write a @newtype@ wrapper and a -- custom 'Result' instance to handle your encoding.