[elephant-devel] Re: traversing btree using multiple indices

[Alex Mizrahi]

 SR> (defclass test-event ()
 SR>   ((date :accessor date-of :initform (now))
 SR>    (name :accessor name-of :initform nil))
 SR>   (:metaclass persistent-metaclass))
 SR> (defun create-date-indexer (a b c)   (values t (date-of c)))
 SR> (defun create-name-indexer (a b c)   (values t (name-of c)))
 SR> (add-index btree
 SR>            :index-name 'date-index
 SR>            :key-form 'create-date-indexer
 SR>            :populate t)

this looks weird. you could just write

(defclass test-event ()
 ((date :accessor date-of :initform (now) :index t) ...

and it will create indices for you automatically.

 SR> what is the best way to map across all added events between a
 SR> particular date range with the name 'Sean'?

that's not union or intersection (at least i won't call them that way) --  
you need an index with custom sorting order to do this.
something like that:

(defun id-and-time (id time) (formant nil "~a,~a" id time))

(add-class-derived-index 'test-event 'by-name-and-date
        '(lambda (e) (id-and-time (name-of e) (date-of e))))

then you'll get sequence like this:

Ross,37343253
Sean,31334456
Sean,32345678
Somebody,30334656

it will work fine as long as name (and date) does not contain #\, -- then 
all entries will same name will be grouped together.
then you can use simple range query:

 (get-instances-by-range 'test-event 'by-name-and-date
                (id-and-time "Sean" start-date)
                (id-and-time "Sean" end-date))

i believe that SQL RDBMS work this way too -- if one needs fast retrieval by 
several keys, he should create index on them. RDMBS knows how to sort 
tuples, though