How to resolve the algorithm Hash join step by step in the Haskell programming language
How to resolve the algorithm Hash join step by step in the Haskell programming language
Table of Contents
Problem Statement
An inner join is an operation that combines two data tables into one table, based on matching column values. The simplest way of implementing this operation is the nested loop join algorithm, but a more scalable alternative is the hash join algorithm. Implement the "hash join" algorithm, and demonstrate that it passes the test-case listed below. You should represent the tables as data structures that feel natural in your programming language. The "hash join" algorithm consists of two steps:
In pseudo-code, the algorithm could be expressed as follows: The order of the rows in the output table is not significant. If you're using numerically indexed arrays to represent table rows (rather than referring to columns by name), you could represent the output rows in the form [[27, "Jonah"], ["Jonah", "Whales"]].
Let's start with the solution:
Step by Step solution about How to resolve the algorithm Hash join step by step in the Haskell programming language
Both snippets provide an implementation of the hash join algorithm in Haskell. The hash join algorithm is used to efficiently perform a join operation between two datasets based on a common key.
The first snippet uses the Data.HashTable.ST.Basic
library to create a hash table and perform the join. The hashJoin
function takes four arguments:
xs
: The first datasetfx
: A function that extracts the key from each element ofxs
ys
: The second datasetfy
: A function that extracts the key from each element ofys
The function first creates an empty hash table ht
and a new mutable reference l
to store the results of the join. It then iterates over the elements of ys
and inserts the key-value pairs into the hash table. The key is extracted using the fy
function, and the value is a list of the corresponding elements from ys
. If the key already exists in the hash table, the corresponding list of values is updated to include the new element.
Next, the function iterates over the elements of xs
and looks up the key in the hash table. If the key exists, the corresponding list of values from ys
is retrieved and combined with the element from xs
using the map
function. The resulting tuples are appended to the mutable reference l
.
Finally, the function returns the contents of the mutable reference l
, which contains the results of the join.
The second snippet uses the Data.Map
library to create a map and perform the join. The mapJoin
function takes four arguments, which are the same as in the first snippet.
The function first creates an empty map yMap
and iterates over the elements of ys
to insert the key-value pairs into the map. The key is extracted using the fy
function, and the value is a list of the corresponding elements from ys
. If the key already exists in the map, the corresponding list of values is updated to include the new element.
Next, the function iterates over the elements of xs
and looks up the key in the map. If the key exists, the corresponding list of values from ys
is retrieved and combined with the element from xs
using the map
function. The resulting tuples are concatenated into a single list.
Finally, the function returns the concatenated list of tuples, which contains the results of the join.
Both snippets provide efficient implementations of the hash join algorithm in Haskell. The first snippet uses a hash table, while the second snippet uses a map. The choice of which snippet to use depends on the performance requirements of the application.
Source code in the haskell programming language
{-# LANGUAGE LambdaCase, TupleSections #-}
import qualified Data.HashTable.ST.Basic as H
import Data.Hashable
import Control.Monad.ST
import Control.Monad
import Data.STRef
hashJoin :: (Eq k, Hashable k) =>
[t] -> (t -> k) -> [a] -> (a -> k) -> [(t, a)]
hashJoin xs fx ys fy = runST $ do
l <- newSTRef []
ht <- H.new
forM_ ys $ \y -> H.insert ht (fy y) =<<
(H.lookup ht (fy y) >>= \case
Nothing -> return [y]
Just v -> return (y:v))
forM_ xs $ \x -> do
H.lookup ht (fx x) >>= \case
Nothing -> return ()
Just v -> modifySTRef' l ((map (x,) v) ++)
readSTRef l
main = mapM_ print $ hashJoin
[(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]
snd
[("Jonah", "Whales"), ("Jonah", "Spiders"),
("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]
fst
{-# LANGUAGE TupleSections #-}
import qualified Data.Map as M
import Data.List
import Data.Maybe
import Control.Applicative
mapJoin xs fx ys fy = joined
where yMap = foldl' f M.empty ys
f m y = M.insertWith (++) (fy y) [y] m
joined = concat .
mapMaybe (\x -> map (x,) <$> M.lookup (fx x) yMap) $ xs
main = mapM_ print $ mapJoin
[(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]
snd
[("Jonah", "Whales"), ("Jonah", "Spiders"),
("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]
fst
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