Aha! I do have indexes on my actual app, but the delete is done with a sub-query, for which it appears the index isn’t being used e.g. “delete from table where id in (select id from Y)”.
Replacing this with multiple single deletes has solved the performance issue.
With the example below, I had no index on id, but as there were only 7 rows in the table I’d assumed it wouldn’t make a difference. I’m slightly surprised that in such a scenario the size of the blob affects the sequential scan of the table so much, but now that I’m aware of it, it’s not an issue. Adding an index to id on the table reduced all the delete times to around 15ms.
what indexes do you have? I would definitely put an index on id. I have blob tables with large image files, and I'm measuring end-to-end delete (from web browser application to server and back) at around 100 ms.
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From: Andrew Lawrenson
To: "email@example.com" <firstname.lastname@example.org>
Sent: Monday, April 20, 2009 2:53:14 AM
Subject: Blobs & Delete speed
I’ve been experiencing a performance problem with deleting blobs in derby, and was wondering if anyone could offer any advice.
This is primarily with 10.4.2.0 under windows and solaris, although I’ve also tested with the new 10.5.1.1 release candidate (as it has many lob changes), but this makes no significant difference.
The problem is that with a table containing many large blobs, deleting a single row can take a long time (often over a minute).
I’ve reproduced this with a small test that creates a table, inserts a few rows with blobs of differing sizes, then deletes them.
The table schema is simple, just:
create table blobtest( id integer generated BY DEFAULT as identity, b blob )
and I’ve then created 7 rows with the following blob sizes : 1024 bytes, 1Mb, 10Mb, 25Mb, 50Mb, 75Mb, 100Mb.
I’ve read the blobs back, to check they have been created properly and are the correct size.
They have then been deleted using the sql statement ( “delete from blobtest where id = X” ).
If I delete the rows in the order I created them, average timings to delete a single row are:
1024 bytes: 19.5 seconds
1Mb: 16 seconds
10Mb: 18 seconds
25Mb: 15 seconds
50Mb: 17 seconds
75Mb: 10 seconds
100Mb: 1.5 seconds
If I delete them in reverse order, the average timings to delete a single row are:
100Mb: 20 seconds
75Mb: 10 seconds
50Mb: 4 seconds
25Mb: 0.3 seconds
10Mb: 0.25 seconds
1Mb: 0.02 seconds
1024 bytes: 0.005 seconds
If I create seven small blobs, delete times are all instantaneous.
It thus appears that the delete time seems to be related to the overall size of the rows in the table more than the size of the blob being removed.
I’ve run the tests a few times, and the results seem reproducible.
So, does anyone have any explanation for the performance, and any suggestions on how to work around it or fix it? It does make using large blobs quite problematic in a production environment…
Many thanks in advance,