Set correct DBDictionary properties for MySQL based on MySQL version
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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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