[TYPO3-core] RFC #13945: Bug: Missing key on l18n parent in tt_content table

[Christian Kuhn]

This is a SVN patch request.

Type: bugfix

Branches: trunk

BT: http://bugs.typo3.org/view.php?id=13945

Problem:
t3lib_BEfunc::translationCount issues a query like 'SELECT COUNT(*) FROM 
tt_content WHERE l18n_parent=123 AND sys_language_uid!=0 AND deleted=0'. 
EXPLAIN shows no usable key for this type of query.

Solution:
Add key on l18n_parent.

Notes:
This is especially useful for the list module which calls this method 
for each listed tt_content record.
I decided against a combined index as there are several other queries 
with l18n_parent in its where part but different additional fields.

This profiling was done with a happy little extension called 
'mf_mysql_profiler' that xclasses t3lib_db to normalize and log queries, 
together with a backend module to analyze them. I encouraged the author 
to upload it to forge.

Regards
Christian
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