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https://issues.apache.org/jira/browse/DERBY-7109?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17306223#comment-17306223
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Holger Rehn commented on DERBY-7109:
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I was able to put together a simplified test reproducing the described problem.
It's enough to simply add a number of rows into a single table and then remove
them all. Turns out my previous assumptions were incorrect and just
side-effects of the way my 1st test worked. The key point seems to be raw table
size. This are my findings:
# if a table grows above a certain size (~2.x GiB), inplace compression seems
only to be able to reclaim the disk space exceeding that boundary
# if a table grows above X times that size, inplace compression can only
reclaim the disk space exceeding X times that limit
# its enough to insert rows until the table reaches that size limit and then
remove them all, I didn't find additional requirements:
** the DB doesn't have to be fragmented
** the table doesn't have to have an index
** the row data doesn't have to be of the same (or a certain) size
** the table doesn't have to have a BLOB column
** it doesn't make any difference if the rows are removed one by one using an
ID or altogether by a global "delete from"
The attached test is using a table with a single BLOB column and larger chunks
of data for performance reasons. I also tried with a table only containing a
few BIGINT columns: The results are comparable and the problem can be
reproduced. But without a BLOB column the test runs ~2 hours instad of a few
seconds just inserting rows to reach the critical DB size. Apart from that the
test should be self-explanatory.
One more thing that caught my eye: If i modify the test to produce less than
the critical amount of table data, SYSCS_INPLACE_COMPRESS_TABLE does even
reclaim *more* disk space than SYSCS_COMPRESS_TABLE. Is that an expected
behaviour? From reading the docs I wouldn't expect that to be possible.
> SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE doesn't work as expected
> ----------------------------------------------------------------
>
> Key: DERBY-7109
> URL: https://issues.apache.org/jira/browse/DERBY-7109
> Project: Derby
> Issue Type: Bug
> Components: Store
> Affects Versions: 10.14.2.0, 10.15.2.0
> Reporter: Holger Rehn
> Priority: Major
> Attachments: DerbyTest.java
>
>
> The SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE procedure seems to be unable to
> reclaim any disk space under almost all circumstances. I have a number of
> existing DBs that I tried to reorganize to reclaim disk space using the
> mentioned procedure. Because that didn't even free the smallest amount of
> space, I started a number of tests and it looks like inplace compression
> simply doesn't work, at all.
> Example:
> One of the DBs uses ~37 GB of disk space and the exact same amount after
> running SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE. After using
> SYSCS_UTIL.SYSCS_COMPRESS_TABLE to reorganize the DB it only uses 20,4 GB of
> disk space. This might be correct, if I correctly understand how inplace
> compression should work - but is not really what I expected. But even after I
> completely deleted a number of tables and removed more than 90% of the rows
> from all other tables of the original DB,
> SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE was unable to reclaim any disk space.
> In contrast, SYSCS_UTIL.SYSCS_COMPRESS_TABLE now reduced the DB size to ~
> 130MB. To carry this to the extreme, I completely removed any data from all
> tables. As expected, procedure SYSCS_UTIL.SYSCS_COMPRESS_TABLE now results in
> an empty DB only taking up ~4 MB of disk space, while
> SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE was still unable to reclaim a
> relevant amount of disk space (only ~ 4,6GB), leaving a completely empty DB
> still occupying more than 32 GB of disk space.
> In all calls to SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE I set PURGE_ROWS,
> DEFRAGMENT_ROWS, and TRUNCATE_END to 1.
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