[elephant-devel] Re: Derived Indicies

[Alex Mizrahi]

 IE> unnecessary.  If you have messages indexed by time than you can simply
 IE> walk the index and filter by user until you have a web page worth of
 IE> messages.

worst case behaviour is very bad -- to prove that there are no messages, it 
will have to scan all of them..
what's even worse, this worst case is quite frequent -- many users actually 
have empty inboxes.

 IE>   A scan, even of an index, may not be fast enough so you could do an
 IE> intersection of user-to values with an ordered list of recent messages.

you mean reading oids from user-to index and scanning/filtering 
modification-time index with them?
this will mean same worst case.
of course sufficiently smart system can see which list is less, but still 
with thousands of messages and thousands of users this is going to be quite 
slow.

 IE> This should perform similarly to a SQL engine which many ORM systems
 IE> use for queries like this.

i thought in SQL DBMS expression like "create index tree11_idx on 
tree11(qi,value)"
builds ordered list (btree) of pairs pretty much as in derived index i've 
described, and this will make lookups on both values fast.