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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Data     Database     Data Science     Frames    
Latest version: v0.2.0.0

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Accessing Postgres in a data frame in Haskell

A library for accessing Postgres tables as in-memory data structures.

This library provides helpers for generating types (at compile time) corresponding to a database schema and 'canned queries' to execute against a database instance. Additionally, it provides utilities to convert plain Haskell records (i.e. the format of query results) to vinyl records (upon which the Frames library is based). Can be used for interactive exploration by loading all data in-memory at once (and converting to a data frame), and also in a constant memory streaming mode.

Usage Example

In this example we assume there is a local Postgres instance with schema and rows given by the small DB-dump present in data/users.sql.

A. Interactive Workflow Steps

  1. Bootstrap database schema: In a new project, assume a file Example.hs is present in the src directory with the code below. You may of course change the string passed to genBeamSchema to match your database instance of interest. ```haskell -- Example.hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeFamilyDependencies #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} module Example where

import qualified Data.Conduit.List as CL import qualified Data.Vinyl.Functor as VF import qualified Frames as F import Frames.SQL.Beam.Postgres

$(genBeamSchema "host=localhost dbname=shoppingcart1")

2. Next, execute `stack build` or `stack ghci`. This compilation step, if completed without any errors, will establish a connection to your database instance of interest, read its schema, generate corresponding Haskell types and put them in a module named `NewBeamSchema` in your `src` directory (the file creation step is also part of the compilation process).

3. Assuming step 2 worked fine for you and you were using the test DB-dump from the `data` folder you should now have a module with code matching that in the `test/NewBeamSchema.hs` file of this repository. In case you used some other database instance of your own, your generated module would look different.
Import this module into `Example`:

-- Example.hs
-- Extensions elided
module Example where

import qualified Data.Conduit.List        as CL
import qualified Data.Vinyl.Functor       as VF
import qualified Frames                   as F
import           Frames.SQL.Beam.Postgres

import NewBeamSchema

$(genBeamSchema "host=localhost dbname=shoppingcart1")
  1. Let's assume the table of interest is Cart_usersT. We want to pull rows from this table into a data frame to explore it interactively from ghci. Note that beam query results are lists of plain Haskell records whereas Frames requires a list of vinyl records. In order to make this conversion, we add the following two invokations of code-generating (Template-Haskell) functions to Example:
-- Example.hs
-- rest of the module elided

import NewBeamSchema

$(genBeamSchema "host=localhost dbname=shoppingcart1")

deriveGeneric ''Cart_usersT
deriveVinyl ''Cart_usersT

...and build your project. This will add some additional code into the Example module. You can inspect this code by adding the appropriate compiler flags to your .cabal file.

  1. Querying the DB: In this step we will execute a SELECT * FROM tbl WHERE... query and convert the results to a data frame. Note that the table declaration (_cart_users) and the database declaration (db) are exported by the NewBeamSchema module. More importantly, these declarations are autogenerated at compile time, so in case new tables are added, the corresponding declarations are automatically available for use.
-- Example.hs
connString :: ByteString
connString = "host=localhost dbname=shoppingcart1"

-- selects 'n' rows from the specified table in the db.
loadRows1 :: Int -> IO [(Cart_usersT Identity)]
loadRows1 n =
  withConnection connString $
    bulkSelectAllRows _cart_users db n

loadRows2 :: Int -> IO [(Cart_usersT Identity)]
loadRows2 n =
  withConnection connString $
    bulkSelectAllRowsWhere _cart_users db n (\c -> (_cart_usersFirst_name c) `like_` "J%")

Notice the lambda passed to bulkSelectAllRowsWhere in loadRows2. This is a 'filter lambda' that forms the WHERE ... part of the SQL query and is executed at the DB-level. We will see how to create our own 'filter lambdas' in another section below. For now, if we were to enter ghci by executing stack ghci after adding the above code:

ghci>res1 <- loadRows1 5
ghci>:t res1
res1 :: [Cart_usersT Identity]
ghci>:t (map createRecId res1)
(map createRecId res1)
  :: [F.Rec
        '["_cart_usersEmail" F.:-> Text,
          "_cart_usersFirst_name" F.:-> Text,
          "_cart_usersLast_name" F.:-> Text,
          "_cart_usersIs_member" F.:-> Bool,
          "_cart_usersDays_in_queue" F.:-> Int]]
ghci>:t (F.toFrame $ map createRecId res1)
(F.toFrame $ map createRecId res1)
  :: F.Frame
          '["_cart_usersEmail" F.:-> Text,
            "_cart_usersFirst_name" F.:-> Text,
            "_cart_usersLast_name" F.:-> Text,
            "_cart_usersIs_member" F.:-> Bool,
            "_cart_usersDays_in_queue" F.:-> Int])
ghci>myFrame = F.toFrame $ map createRecId res1
ghci>:set -XTypeApplications
ghci>:set -XTypeOperators
ghci>:set -XDataKinds
ghci>miniFrame = fmap (F.rcast @'["_cart_usersEmail" F.:-> Text, "_cart_usersDays_in_queue" F.:-> Int]) myFrame
ghci>mapM_ print miniFrame
{_cart_usersEmail :-> "[email protected]", _cart_usersDays_in_queue :-> 1}
{_cart_usersEmail :-> "[email protected]", _cart_usersDays_in_queue :-> 42}
{_cart_usersEmail :-> "[email protected]", _cart_usersDays_in_queue :-> 1}
{_cart_usersEmail :-> "[email protected]", _cart_usersDays_in_queue :-> 42}
{_cart_usersEmail :-> "[email protected]", _cart_usersDays_in_queue :-> 1}

We could have used loadRows2 in place of loadRows1 in order to have the WHERE ... clause executed at the DB-level. Note that in the above, once the query results are converted to a data frame, you're free to play with the frame in anyway, just like you would for a data frame created from a CSV.

B. Streaming Workflow Steps

Once you're done working with a small subset of data, and would like to scale up your analysis by looking at a larger-subset-of/complete data, then it's time to look at writing your own conduit to process incoming rows from the DB.

1 - 4: Same as 'Interactive Workflow Steps'

  1. Writing your own streaming pipeline:

Consider the following:

streamRows :: IO ()
streamRows = do
  res <-  withConnection connString $
            streamingSelectAllPipeline' _cart_users db 1000 (\c -> (_cart_usersFirst_name c) `like_` "J%") $
              (CL.map (\record -> F.rcast @["_cart_usersEmail" F.:-> Text, "_cart_usersIs_member" F.:-> Bool] record))
  mapM_ print res

In the above, we select all rows from the specified table that match a certain pattern ("J%"), then the function streamingSelectAllPipeline' converts the query results to vinyl records inside a conduit and sends it downstream, where we can operate on its output. Here, specifically, we do a column subset of the output using rcast, and CL.map applies rcast to every incoming row and sends it downstream, where the result gets returned. We then print the list of vinyl records.

In order to write your own conduit, all you need to know is that internally the conduit flow is as follows:

(\c -> runConduit $ c .| CL.map createRecId
                      .| recordProcessorConduit
                      .| CL.take nrows)

In the above, you supply the recordProcessorConduit to the streamingSelectAllPipeline' function which takes a vinyl record as input and sends it downstream to the CL.take. Note that in all functions in the Frames.SQL.Beam.Postgres.Streaming module, you need to specify the number of rows you want to return (this is an upper bound of sorts, the actual number of rows returned depends on the amount of data present in your database).

A Note on 'Canned Queries' and 'Filter Lambdas'

There are three things needed to execute a canned query (SELECT * FROM tbl WHERE ...):

  • PostgresTable a b: auto generated by BeamSchemaGen module
  • PostgresDB b: auto generated by BeamSchemaGen module
  • PostgresFilterLambda a s: The WHERE... clause. All filter lambdas are of the form: haskell (\tbl -> (_fieldName tbl) `op` constant) or haskell (\tbl -> (_fieldName1 tbl) `op` (_fieldName2 tbl)) In the above op can be one of : [==., /=., >., <., <=., >=., between_, like_, in_ ] (some of these are not be applicable to the second case). You may use (&&.) and (||.) to combine expressions inside the lambda. To see some actual examples of 'filter lambdas', check out test/LibSpec.hs in this repository.

Background Reading: