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https://issues.apache.org/jira/browse/OPENJPA-564?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12587222#action_12587222
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Jason Zheng commented on OPENJPA-564:
-------------------------------------
The patch is against openjpa version:
OpenJPA 1.1.0-SNAPSHOT
version id: openjpa-1.1.0-SNAPSHOT-r420667:634150M
Apache svn revision: 420667:634150M
> 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
> Fix For: 1.1.0
>
> Attachments: openjpa-1.1.0-634377.patch
>
>
> 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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