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From <g.h...@aurenz.de>
Subject Derby Scheduler and FETCH FIRST question
Date Wed, 24 Jan 2018 12:45:37 GMT
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

 


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