As soon as you consider that a delete operation will need to update all the index the deleted entry uses, you see that you can benefit from having such reverse index : you don't have to grab the entry from the backend, as you already have it's ID, which is all what you need. Thus, a delete operation is just about doing :
- get the entry ID
- for each index, if we have an <Entry ID> stored, then grab the associated values, and for each value, delete them from the forward index.
- delete the entry from the MT
Now, it's not enough to say 'kill the reverse index !'. There is one more reason why we want to have this reverse index, the question is : does it brings a lot of benefit ?
So why do we need this reverse index ? We discussed about it with Stefan, and here is what it is used for : Suppose you have a search request with a filter like (&(ObjectClass=XXX)(cn=YYY)).
The search engine will evaluate the number of entries each of those filters node will get back. Suppose it's N for the OC filter, and M for the cn filter. Let's say that N > M.
Now, we will loop on the M entries to check if they fit the OC filter.
How do we do that ? The first approach would be to grab the entry, and check in memory of the filter (ObjectClass=XXX) match the entry. If not, we ditch the entry, otherwise, itas a valid candidate.
The second option is to use the reverse index : we have the entry ID (we havdn't grabbed the entry from disk yet), and we can see if the OC table contains a reference for this entry ID. If not, we can move to the next entry ID. Otherwise, we can grab the entry and return it.
Obviously there is some potential for a speedup. Now, let's consider the cases where we benefit from not grabbing the entry.
1) We grab all the entry using the smallest set selected with the filters
- Fast entry filtering, it's just a question on applying the filters on the entry
- It can be used very late, as we have already grabbed the entry. The entry selection can not only be based on the filter, but can also be combined with virtual attributes which has been added on the grabbed entry
- That means we can move the search engine out of the backend, and have it working on the top of the Interceptor chain
- We may read way more entries than necessary
2) we just get the Entry ID and use the reverse index to select the entries
- we don't grab the entries unless absolutely necessary
- we may have to check in many indexes (as many as we have exprNodes in the filter expression, assuming that each of them are indexed), which means lot of O(log(N)) operations on these index (assuming that all those index are in memory, otherwise, if we hit the disk, the benefit is null)
- as soon as we have one single non indexed exprNode in the filter we have to check, then we will have to grab the entry anyway. The benefit is that we may save some disk access.
- of course, we have to maintain all the reverse indexes.
- if the exprNode is associated with an attribute not present in te entry, we do a lookup in the index for nothing
- if the intersection between the two exprNode is big (ie Nb(ObjectClass=XXX) inter Nb(cn=YYY)), then the gin will be low, and if this itersection is small, then it's likely that the smallest set has been already selected as the main index to use to grab entries, thus leading to a small number of entries to grab.
So here is what I suggest :
- get rid of those reverse index
- or, at least, make it optionnal