Re: (ITS#6696) back-sql and pagedResultsControl can be extremely heavy due to no LIMIT

[hyc@symas.com]

masarati@aero.polimi.it wrote:
> Andrew.Gray@unlv.edu wrote:
>> Full_Name: Andrew Gray
>> Version: 2.4.17
>> OS: Debian 5.0
>> URL: ftp://ftp.openldap.org/incoming/
>> Submission from: (NULL) (131.216.14.1)
>>
>>
>> On receiving LDAP queries with a pagedResultsControl (in this case with a size
>> of 250), back-sql generates an extremely inefficient query for every iteration
>> in the form of:
>>
>> SELECT DISTINCT ldap_entries.id,people.local_id,text('UNLVexpperson') AS
>> objectClass,ldap_entries.dn AS dn FROM ldap_entries,people,ldap_entry_objclasses
>> WHERE people.local_
>> id=ldap_entries.keyval AND ldap_entries.oc_map_id=1 AND upper(ldap_entries.dn)
>> LIKE upper('%'||'%OU=PEOPLE,DC=UNLV,DC=EDU') AND ldap_entries.id>250 AND (2=2 OR
>> (ldap_entries.id=ldap_entry_objclasses.entry_id AND ldap_entry_objclasses.oc_
>> name='UNLVexpperson'))
>>
>> (this repeats for id>250, id>500, id>750, etc. etc.)
>>
>> Ideally (IMO) there really should be a SQL LIMIT applied here, as in this case
>> slapd gets back a few tens of thousands of rows on every iteration, and the
>> memory usage explodes and eventually gets killed.
>
> Using back-sql on large databases along with pagedResult control is not
> advisable.  Limiting the number of entries returned by each query is not
> viable as well, since some entries might not mathc the LDAP filter, or
> ACLs or so, possibly leading to less than pageSize entries returned
> within one page.  PagedResults could be removed from back-sql, and dealt
> with by an overlay that simply pages results returned by back-sql in a
> single internal search; probably this is the preferable approach, since
> it would also result in a reduction of the complexity of back-sql.
> However, I have little interest in improving back-sql, so patches are
> welcome, as usual...

The sssvlv overlay already intercepts pagedResults requests if they occur in 
combination with the Sort control. It would be trivial to extend it to always 
intercept pagedResults, and then we can rip the paging support out of each of 
the backends. (Of course, there's a marginal efficiency advantage to letting 
back-bdb/hdb do its own paging. A configurable option might be best.)

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/