serialization of an Object[] array and LIMIT option implementation

[Maciej Regulski <mr...@go2.pl>]

Hi,
I'm very beginer to OJB. I've got two questions.
Are you going to fix org.apache.ojb.broker.core.PersistenceBrokerImpl class
code to suport storing m:n relations implemented as an Object[] array?
I tried to patch it my self and finaly I got to this point when I had to
convert array of persistent object to Vector to avoid
java.lang.ClassCastException, but I don't think it is a good solution.

PersistenceBrokerImpl::storeCollections:

  [..]
                if (cds.isMtoNRelation())
                {
                    currentMtoNKeys = getMtoNImplementor(cds, obj);
                    // delete unused m:n implementors
                   //try-catch implementation added by Mathias for escape
from ClassCastException
                   try {
                        deleteMtoNImplementor(cds, obj, (Collection)col,
currentMtoNKeys);
                   } catch (ClassCastException cce) {
                        if (col.getClass().isArray()) {
                            System.out.println("::IF ENTERED");
                            deleteMtoNImplementor(cds, obj,
(Collection)pl.ojb.tools.Array2Vector.Convert((Object[])col),
currentMtoNKeys);
                        }
                   }
   [..]

And second question is connected with buiding SELECT statement. I know that
LIMIT option is specific for each database, but getting whole record set
from db server every time is a disaster. Espetially when you have thousands
of records in your sql tables. And again I had to interfere in OJB source
code:
org.apache.ojb.broker.accesslayer.sql.SqlSelectStatement:getStatement

 [..]
        appendOrderByClause(orderByFields, orderByColumnNumbers, stmt);
 /**
  * PostgreSQL only LIMIT implementation
  * if the query has specified a start at index, and an end at index move to
the start at index.
  */
 int startAt = query.getStartAtIndex();
 int endAt = query.getEndAtIndex();
 if (startAt > 0) {
            stmt.append(" OFFSET "+String.valueOf(startAt));
 }
 if (endAt > 0) {
            int numberOfObjectsToFetch = endAt;
            if (startAt > 0) numberOfObjectsToFetch =
numberOfObjectsToFetch - startAt;
            stmt.append(" LIMIT "+String.valueOf(endAt));
 }
 return stmt.toString();

It works fine for me, but now I had to change db server to MySQL. :>


So is there any chance to have this issues solved with next version of
OJB???
Rgds
Mathias <ma...@irc.pl>


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Re: serialization of an Object[] array and LIMIT option implementation

[Jakob Braeuchi <jb...@gmx.ch>]

hi maciej,

the LIMIT is supported by use of the PagingIterator. OJB still reads the 
whole resultset but positions the cursor to the desired position. see 
Query.setStartAt / setEndAt.
i also experimented with SQL-Paging (see the Platforms). this solution 
worked, but the drawback is the way OJB supports extents: 1 select for 
each extent.

jakob

Maciej Regulski wrote:
> Hi,
> I'm very beginer to OJB. I've got two questions.
> Are you going to fix org.apache.ojb.broker.core.PersistenceBrokerImpl class
> code to suport storing m:n relations implemented as an Object[] array?
> I tried to patch it my self and finaly I got to this point when I had to
> convert array of persistent object to Vector to avoid
> java.lang.ClassCastException, but I don't think it is a good solution.
> 
> PersistenceBrokerImpl::storeCollections:
> 
>   [..]
>                 if (cds.isMtoNRelation())
>                 {
>                     currentMtoNKeys = getMtoNImplementor(cds, obj);
>                     // delete unused m:n implementors
>                    //try-catch implementation added by Mathias for escape
> from ClassCastException
>                    try {
>                         deleteMtoNImplementor(cds, obj, (Collection)col,
> currentMtoNKeys);
>                    } catch (ClassCastException cce) {
>                         if (col.getClass().isArray()) {
>                             System.out.println("::IF ENTERED");
>                             deleteMtoNImplementor(cds, obj,
> (Collection)pl.ojb.tools.Array2Vector.Convert((Object[])col),
> currentMtoNKeys);
>                         }
>                    }
>    [..]
> 
> And second question is connected with buiding SELECT statement. I know that
> LIMIT option is specific for each database, but getting whole record set
> from db server every time is a disaster. Espetially when you have thousands
> of records in your sql tables. And again I had to interfere in OJB source
> code:
> org.apache.ojb.broker.accesslayer.sql.SqlSelectStatement:getStatement
> 
>  [..]
>         appendOrderByClause(orderByFields, orderByColumnNumbers, stmt);
>  /**
>   * PostgreSQL only LIMIT implementation
>   * if the query has specified a start at index, and an end at index move to
> the start at index.
>   */
>  int startAt = query.getStartAtIndex();
>  int endAt = query.getEndAtIndex();
>  if (startAt > 0) {
>             stmt.append(" OFFSET "+String.valueOf(startAt));
>  }
>  if (endAt > 0) {
>             int numberOfObjectsToFetch = endAt;
>             if (startAt > 0) numberOfObjectsToFetch =
> numberOfObjectsToFetch - startAt;
>             stmt.append(" LIMIT "+String.valueOf(endAt));
>  }
>  return stmt.toString();
> 
> It works fine for me, but now I had to change db server to MySQL. :>
> 
> 
> So is there any chance to have this issues solved with next version of
> OJB???
> Rgds
> Mathias <ma...@irc.pl>
> 
> 
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> To unsubscribe, e-mail: ojb-dev-unsubscribe@db.apache.org
> For additional commands, e-mail: ojb-dev-help@db.apache.org
> 
> 


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