Hello everyone,
I'm using Apache Derby v10.14.1.0 and having some problems using the FETCH FIRST clauses. https://db.apache.org/derby/docs/10.14/ref/rrefsqljoffsetfetch.html I'm accessing the database using the Derby Embedded driver. I have a table which contains some indexes as well as some fields and a BLOB field. The table is somewhat big (means many rows, ~13 GB). I'm using a query like this (timestamp has an index): SELECT * FROM history ORDER BY timestamp DESC FETCH FIRST 10 ROWS ONLY The query takes ages (about 27 minutes for that ~13 GB table) and I can see how Derby slowly fills up my harddisk. And a look in the "tmp" folder of the database shows several ".tmp" files. First I get several files having 10 MB, then I get two big files having 5 GB, then the 10 MB files are deleted, then the 5 GB files are deleted and finally I get the result. As I thought something is wrong with my application I also did the same query on the same database and table using SQuirreL v3.8.1. But the result is the same. I would have expected that the scheduler of Derby would first look at the timestamp column / index (which should be sorted), taking the first 10 values from there and finally reading the first 10 rows matching these values. Instead it seems that it first processes the " SELECT * FROM history" part (as memory is not sufficient it swaps it to the harddisk), orders it and takes the first 10 elements. Is that correct? And if that is correct, where is the benefit of FETCH FIRST - beside that maybe not that much data is transferred (maybe only interesting if you use Derby not by the Embedded Driver because of the TCP/IP connection)? Regards, Gerrit
