Обсуждение: effects of nullifying bytea column on storage
Hi:
psql (11.5, server 11.3) on linux
I have a table with a bytea column which, of course, contains binary data. After 60 days, I no longer need the binary data but want to retain the rest of the record. Of course it's easy to just update the bytea column to null for the older records. But I can almost imagine this record on disk with a big "hole" in the middle where the bytea data used to be. Is there a PG daemon (the vacuum ?) that will "heal the hole" in time?
Thanks !
On Wednesday, May 11, 2022, David Gauthier <dfgpostgres@gmail.com> wrote:
Hi:psql (11.5, server 11.3) on linuxI have a table with a bytea column which, of course, contains binary data. After 60 days, I no longer need the binary data but want to retain the rest of the record. Of course it's easy to just update the bytea column to null for the older records. But I can almost imagine this record on disk with a big "hole" in the middle where the bytea data used to be. Is there a PG daemon (the vacuum ?) that will "heal the hole" in time?
The freshly written record will not have a hole - either by virtue of variable width fields taking up basically their actual data space and also the fact that null is treated specially in the record format.
For the old row, yes you should read up on the how and why of the vacuum command.
David J.
Doesn't vacuum run automatically (or can it be set to run automatically) ?
On Wed, May 11, 2022 at 8:05 PM David G. Johnston <david.g.johnston@gmail.com> wrote:
On Wednesday, May 11, 2022, David Gauthier <dfgpostgres@gmail.com> wrote:Hi:psql (11.5, server 11.3) on linuxI have a table with a bytea column which, of course, contains binary data. After 60 days, I no longer need the binary data but want to retain the rest of the record. Of course it's easy to just update the bytea column to null for the older records. But I can almost imagine this record on disk with a big "hole" in the middle where the bytea data used to be. Is there a PG daemon (the vacuum ?) that will "heal the hole" in time?The freshly written record will not have a hole - either by virtue of variable width fields taking up basically their actual data space and also the fact that null is treated specially in the record format.For the old row, yes you should read up on the how and why of the vacuum command.David J.
On Wednesday, May 11, 2022, David Gauthier <dfgpostgres@gmail.com> wrote:
Doesn't vacuum run automatically (or can it be set to run automatically) ?
David J.
On Wed, 11 May 2022 at 20:02, David Gauthier <dfgpostgres@gmail.com> wrote: > I have a table with a bytea column which, of course, contains binary data. After 60 days, I no longer need the binarydata but want to retain the rest of the record. Of course it's easy to just update the bytea column to null for theolder records. But I can almost imagine this record on disk with a big "hole" in the middle where the bytea data usedto be. Is there a PG daemon (the vacuum ?) that will "heal the hole" in time? We have a similar situation, and run a weekly task that backs up the table, sets the outdated records to NULL, truncates the original table, then copies the records back in. With this approach the disk space is reclaimed immediately. The table is only written to by automated processes, so we have a weekly maintenance window where these aren't running. This might be a possible approach.