Index seems to return wrong amount of candidate causing really poor search performance
Considering the following assumptions;
- OpenLDAP version 2.4.51
- attributes objectClass and abc are indexed based on equality
- the EQUALITY of attribute abc is based on distinguishedNameMatch
- The database contains roughly 2 million entries
- 2 entries have defined the attribute abc with a dn value cn=foo,dc=bar and objectClass=someClass
- 2 entries have defined the attribute abc with a dn value cn=bar,dc=baz and objectClass=someClass
Now, the issue started with really slow search performance using objectClass=someClass & abc=cn=foo,dc=bar as filter criteria. Debugging a while seems to indicate that the objectClass filter returns roughly 2 million entries as candidates. Now, one would expect that the second filter would return only the 2 potential candidates from the abc index, or a subset of the whole database but this is not the case. The second filter also returns nearly the whole database entries as potential candidates and causes really slow query performance. Interestingly, this only occurs when attribute abc has value cn=foo,dc=bar, but for some reason for the entry having attribute abc with value cn=bar,dc=baz the query returns immediately. In both cases, the actual entries matching the search return immediately but for the problematic search "(&(objectClass=someClass)(abc=cn=foo,dc=bar))", the completion of the search takes a long time (around 15 seconds to be precise).
The issue started suddenly and wasn't a degradation of query performance over time.
Few things I have tried
- Rebuilt the whole database again
- Reindex the existing database again
- Testing with bdb and mdb as backends
- Increased cache sizes for bdb to hold the whole database in cache
- For bdb adjust the page size of the indexes according to suggestion by db_tuner
- Change the order of the filters
None of these made any difference. At the moment, there does not seem to be any good options to try. Any ideas or help would be greatly appreciated!