Updating where id in stored procedure - enormous amount of fetches

UPDATE TABLE SET SOMETHING = 1 WHERE ID IN (SELECT ID FROM STORED_PROCEDURE)

Records in TABLE = 2100

Records from stored procedure = 50

This statement cousing enormous amount of fetches (31M!) on firebird 2.5 server. Why? Shouldn't it first select id's from stored procedure and then put it in where clau开发者_如何学运维se? What should I do to make it work?

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Try something like this:

declare variable myid integer;    
begin
    for
        select id from stored_procedure into :myID
    do begin
      update table set something = 1 where table.id = :myID;
    end
end