Optimize query to determine whether a frontend user has access to content elements or pages
The WHERE part of the query to determine whether a frontend user has access to a particular element (tt_content in this example) looks like this:
(tt_content.fe_group='' OR tt_content.fe_group IS NULL OR tt_content.fe_group='0' OR FIND_IN_SET('0',tt_content.fe_group) OR FIND_IN_SET('-2',tt_content.fe_group) OR FIND_IN_SET('2',tt_content.fe_group))
It could look like the following (to simplify the OR parts for each group):
(pages.fe_group='' OR pages.fe_group IS NULL OR pages.fe_group='0' OR pages.uid IN (SELECT uid_local FROM pages_fe_groups_rel_mm WHERE uid_foreign IN (0,-2,2)))
(issue imported from #M15985)
#2 Updated by Björn Pedersen about 9 years ago
from: http://dev.mysql.com/doc/refman/5.1/en/subquery-restrictions.html (same also for 5.5) #
Subquery optimization for IN is not as effective as for the = operator or for the IN operator.
A typical case for poor IN subquery performance is when the subquery returns a small number of rows but the outer query returns a large number of rows to be compared to the subquery result.
The problem is that, for a statement that uses an IN subquery, the optimizer rewrites it as a correlated subquery. Consider the following statement that uses an uncorrelated subquery:
SELECT ... FROM t1 WHERE t1.a IN (SELECT b FROM t2);
The optimizer rewrites the statement to a correlated subquery:
SELECT ... FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t2.b = t1.a);
If the inner and outer queries return M and N rows, respectively, the execution time becomes on the order of O(M×N), rather than O(M+N) as it would be for an uncorrelated subquery.
An implication is that an IN subquery can be much slower than a query written using an IN operator that lists the same values that the subquery would return. #
#6 Updated by Alexander Opitz over 4 years ago
- Status changed from Needs Feedback to Closed
No feedback within the last 90 days => closing this issue.
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