CODEWITHSUNDEEP
databasehaskellcursor
Ralph
Ralph

Reputation: 32294

Processing a database cursor in Haskell

In a related Scala question, I asked the following:

When I need to read millions of database rows from a PostgreSQL database using the JDBC driver, I always use a cursor, otherwise I will get an OutOfMemoryError. Here is the pattern (pseudocode) that I use:

begin transaction
execute("declare cursor...")
while (true) {
  boolean processedSomeRows = false
  resultSet = executeQuery("fetch forward...")
  while (resultSet.next()) {
    processedSomeRows = true
    ...
  }
  if (!processedSomeRows) break
}
close cursor
commit

How can this be done in idiomatic Haskell?

Upvotes: 3

Views: 253

Answers (1)

Fedor Gogolev
Fedor Gogolev

Reputation: 10551

There is quite new concept for dealing with streams like sql cursor: iteratees, or enumerator, or conduit. For example, in terms of the conduit library, from Persistent Book:

runResourceT $ withStmt "declare cursor..." []
    $$ mapM_ doSomethingWithSingleResult

withStmt "declare cursor..." [] creates source with rows, mapM_ doSomethingWithSingleResult creates sink for process single rows, and $$ connects source with sink.

Upvotes: 3

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