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From "Jason Zheng (JIRA)" <j...@apache.org>
Subject [jira] Created: (OPENJPA-564) Set correct DBDictionary properties for MySQL based on MySQL version
Date Wed, 09 Apr 2008 03:25:24 GMT
Set correct DBDictionary properties for MySQL based on MySQL version
--------------------------------------------------------------------

                 Key: OPENJPA-564
                 URL: https://issues.apache.org/jira/browse/OPENJPA-564
             Project: OpenJPA
          Issue Type: Improvement
          Components: jdbc
         Environment: Microsoft Windows XP SP2, Java version = 1.5.0 with MySQL 5.x Database.
            Reporter: Jason Zheng
            Priority: Minor


MySQL has many improvements since version 4.1. We should update default values of MySQLDictionary
properties to reflect these changes. Ideally we should be able to set these properties correctly
based on MySQL version, so the users don't need override "openjpa.jdbc.DBDictionary" property
in persistence.xml.

These properties include:
1) DriverDeserializesBlobs (current default value: true) 
We added this flag to MySQLDictionary in Kodo4.1 since most MySQL drivers then automatically
deserialized BLOBs on calls to ResultSet.getObject. This is actually a bug in MySQL, and that
bug(#25328 in MySQL bug database) is already fixed in 5.x driver. Please see http://dev.mysql.com/doc/refman/5.1/en/cj-news-5-0-6.html
for details.

2) SupportSubselect (current default value: false)
MySQL supports sub-select since version 4.1. 

3) AllowsAliasInBulkClause (current default value: false)
This property is only used in DBDictionary.toBulkOperation():
line 1874:
        // if there is only a single table in the select, then we can
        // just issue a single DELETE FROM TABLE WHERE <conditions>
        // statement; otherwise, since SQL doesn't allow deleting
        // from one of a multi-table select, we need to issue a subselect
        // like DELETE FROM TABLE WHERE EXISTS
        // (SELECT 1 FROM TABLE t0 WHERE t0.ID = TABLE.ID); also, some
        // databases do not allow aliases in delete statements, which
        // also causes us to use a subselect
        if (sel.getTableAliases().size() == 1 && supportsSubselect
            && allowsAliasInBulkClause) {
            SQLBuffer from;
            if (sel.getFromSelect() != null)
                from = getFromSelect(sel, false);
            else
                from = getFrom(sel, false);

            sql.append(from);
            appendUpdates(sel, store, sql, params, updateParams,
                allowsAliasInBulkClause);

            SQLBuffer where = sel.getWhere();
            if (where != null && !where.isEmpty()) {
                sql.append(" WHERE ");
                sql.append(where);
            }
            return sql;
        }  

It confuses me why we need this property (and supportsSubselect) as conditions for single
table update/delete. I believe we can set this property to true if we use MySQL version 4.1
or later. Otherwise If we set  supportSubselect to true, we will run into problems described
in
http://issues.apache.org/jira/browse/OPENJPA-459. Note Patrick had a comment for that issue:
"I believe that the first query (which only involves one table) is due to a fairly trivial-to-resolve
limitation of DBDIctionary: currently, if SupportsSubselect is true and AllowsAliasInBulkClause
is false, OpenJPA always uses a subselect, even when it would be valid to just omit the aliases."


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