Of the many doubts that I have, I do not understand the difference between the tables reserves and old_reserves.
The 'reserves' table contains "active" holds. Holds which have not yet been fulfilled or canceled. When a hold has been fulfilled it is moved to the 'old_reserves' table for historical/reporting purposes. You would refer to 'old_reserves' if you were seeking statistics about holds processed in the past.
If I need to get a list of all the holds, for which the books are currently not available in the library, which table would I refer to?
Since you say "currently," that means the 'reserves' table. Note that a similar relationship exists between 'issues' and 'old_issues.' Similarly, 'borrowers' and 'deletedborrowers,' 'biblio' and 'deletedbiblio.' http://schema.koha-community.org/ -- Owen -- Web Developer Athens County Public Libraries https://www.myacpl.org