Re: [Haskelldb-users] HaskellDB efficience

[Krasimir A. <ka2...@ya...>]

--- Bjorn Bringert <d00...@dt...> wrote:
> Hi Krasimir,
> 
> the problem is in the code that you use to print the
> query. You are not 
> actually seeing the query that gets executed since
> the query is not 
> optimized yet. If you use
> Database.HaskellDB.showSql, you will see the 
> optimized query.
> 
> /Bjorn

Thanks! With showSql the things are much better. Now I
got:

SELECT DISTINCT docDate
FROM  documents as T1

This is better but why is there DISTINCT keyword. Can
I get the query without it? The SQL script is even
worse when I have two tables in query:

[****** Haskell ******]
import Database.HaskellDB
import Database.HaskellDB.Query
import Database.HaskellDB.Sql
import Database.HaskellDB.GenericConnect
import Text.PrettyPrint
import Testdb.Documents

main = do
  let primQuery = do
	       docs1 <- table documents
	       docs2 <- table documents
	       restrict (docs1!xid .==. docs2!xid)
	       project (docDate << docs1!docDate #
	                docDate << docs2!docDate)
  putStrLn (render (showSql primQuery))

[****** SQL ******]
SELECT DISTINCT docDate1 as docDate,
                docDate1 as docDate
FROM  (SELECT DISTINCT id as id2,
                       docDate as docDate2
       FROM  documents as T1) as T1,
      (SELECT DISTINCT id as id1,
                       docDate as docDate1
       FROM  documents as T1) as T2
WHERE (id1 = id2)

There are two derived tables, three distincts and one
cross join. Can I get the optimal query?

SELECT T1.docDate as docDate1,
       T1.docDate as docDate2
FROM documents as T1
JOIN documents as T2 on T1.id = T2.id

Regards,
  Krasimir



		
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