Thread: [GENERAL] Question about TOAST table - PostgreSQL 9.2

[GENERAL] Question about TOAST table - PostgreSQL 9.2

From

Patrick B

Date:

28 February 2017, 01:44:13

Hi all.

I have a database which is 4TB big. We currently store binary data in a bytea data type column (seg_data BYTEA). The column is behind binary_schema and the files types stored are: pdf, jpg, png.

Getting the schema binary_schema size:

SELECT pg_size_pretty(pg_database_size('live_database')) As fullprod,
pg_size_pretty(CAST(pg_database_size('live_database') - (SELECT SUM(pg_total_relation_size(table_schema || '.' || table_name) )
FROM information_schema.tables WHERE table_schema = 'binary_schema') As bigint)) As tobebackedup_size,
pg_size_pretty(CAST((SELECT SUM(pg_total_relation_size(table_schema || '.' || table_name) )
FROM information_schema.tables
WHERE table_schema = 'binary_schema') As bigint) ) As junk_size;

fullprod tobebackedup_size junk_size
-------- ----------------- ---------
4302 GB 489 GB 2813 GB

On my database, using pgadmin, I can see a lot of pg_tast_temp_* and pg_temp_* table.

I understand the TOAST code is triggered when a row is wider than the TOAST_TUPLE_THRESHOLD [1]. I also understand the only way to shrink toast table is by using a vacuum full or even pg_dump.

Questions:

1 - If I take out 500GB of bytea data ( by updating the column seg_data and setting it to null ), will I get those 500GB of free disk space? or do I need to run vacuum full or either pg_dump?

2 - If I choose going ahead with VACUUM FULL, I have 3 streaming replication slaves, Will I need to run the vacuum full on them too?

3 - [2] vacuum full needs some free disk space as same size as the target table. It locks the table (cannot be used while running vacuum full) and a REINDEX might be needed after. AM I right?

Thanks in advanced for your help.

Patrick

[1] https://www.postgresql.org/docs/9.2/static/storage-toast.html

[2] https://wiki.postgresql.org/wiki/VACUUM_FULL

Re: [GENERAL] Question about TOAST table - PostgreSQL 9.2

From

Albe Laurenz

Date:

28 February 2017, 20:33:24

Patrick B wrote:
> I have a database which is 4TB big. We currently store binary data in a bytea data type column
> (seg_data BYTEA). The column is behind binary_schema and the files types stored are: pdf, jpg, png.

> Questions:
> 
> 1 - If I take out 500GB of bytea data ( by updating the column seg_data and setting it to null ), will
> I get those 500GB of free disk space? or do I need to run vacuum full or either pg_dump?

You'll need VACUUM (FULL) or dump/restore.

> 2 - If I choose going ahead with VACUUM FULL, I have 3 streaming replication slaves, Will I need to
> run the vacuum full on them too?

No, and indeed you cannot.
The changes made by VACUUM on the primary will be replicated.

> 3 - [2] vacuum full needs some free disk space as same size as the target table. It locks the table
> (cannot be used while running vacuum full) and a REINDEX might be needed after. AM I right?

It locks the table for all concurrent access, but a REINDEX is not necessary, as the
indexes are rewritten as well.

Yours,
Laurenz Albe