On 1/26/18 12:11 AM, [email protected] wrote:
Hello Rick,
and thanks for your reply.
I will try what you've written as soon as I have time for it.
Unfortunately I already switch to another project and I don't know
when I will get the time to have a look on this again (blame my
superiors ;-) ).
2) is surely a good idea.
1) Did you also read my 2nd mail?
I just responded to that 2nd message. Can you share the CREATE TABLE and
CREATE INDEX statements associated with this table? They may provide
some further clues.
Thanks,
-Rick
I also tried using a subselect, so I have a WHERE clause. I had the
same idea as you that the scheduler might not recognize the ORDER BY
and FETCH FIRST.
It was faster, but still not what I would have expected. I've worked a
lot with Borland Interbase / Firebird, MySQL and especially with
PostgreSQL.
And PostgreSQL would have done a lot faster than this.
By the way: PostgreSQL also has a more easy to use approach in aspect
of analysis: https://www.postgresql.org/docs/9.6/static/using-explain.html
Would be great of Derby would offer something similar.
Regards,
Gerrit
*Von:*Rick Hillegas [mailto:[email protected]]
*Gesendet:* Freitag, 26. Januar 2018 00:39
*An:* [email protected]
*Betreff:* Re: Derby Scheduler and FETCH FIRST question
On 1/24/18 4:45 AM, [email protected] <mailto:[email protected]> wrote:
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
Hi Gerrit,
Can you share table and index DDL for this problem as well as the
query plan which Derby chose for the query? See the section on
"Working with RunTimeStatistics" in the Derby Tuning Guide:
http://db.apache.org/derby/docs/10.14/tuning/index.html
It may be that Derby did not choose the index. That in turn, may have
happened for 2 reasons:
1) You're selecting all of the columns in the table and there is no
filtering WHERE clause. That reduces the likelihood that Derby will
pick an indexed access path since the optimizer sees this as a full
table scan.
2) I don't think that any optimizer support was built for the FETCH
FIRST clause. That's worth filing a performance bug for. I think that
the FETCH FIRST clause is only applied at execution time in order to
short-circuit the number of rows which are returned.
Thanks,
-Rick
