> select count(*) from person where BirthDate<= :EndDate and (DeathDate>
> :EndDate or DeathDate is null)
>
> This SQL statement works well but it is very slow because it must read
> all records, or at least, from the BirthDate to the current date if
> the table is indexed by BirthDate.
>
> It can take a lot of time, from 30 seconds to some minutes depending
> on computer.

Have you tried creating a compound index, and testing how much faster than runs? Are you always
querying by both start and end as in your example above?

create index ix_lifetime on person (deathdate, birthdate);

Also, that 'or' could be causing some problems for optimizing the index usage, you might try
breaking it up into two searches (if you weren't doing a straight 'count', you could use this same
technique with a union instead):

select
/* this one could benefit from a compound index on (BirthDate, DeathDate) or (DeathDate,
BirthDate) */
(select count(*) from person where BirthDate <= :EndDate and DeathDate > :EndDate)
+
/* this one could benefit from a compound index on (DeathDate, BirthDate) */
(select count(*) from person where BirthDate <= :EndDate and DeathDate is null)
from rdb$database;

(Do you want to support the case where BirthDate is unknown, and treated as "since forever"?)

I may be a bit fuzzy on that compound indexing ... wait for confirmation from someone else on the
list or just try it out, I guess.

-Philip