I'm constantly caught by surprised how certain operations generate me huge amount of WAL files.
I want these WAL files for point in time recovery (I also perform a nightly full dump in addition) so the basic functionality provided is wanted and I don't want to change that (i.e. I'm not searching for a way to turning WAL archives off, etc.)
Using Postgres 9.5 with these settings:
wal_level = archive checkpoint_timeout = 20min max_wal_size = 1GB min_wal_size = 80MB archive_command = 'test ! -f /backup/wal/%f && cp %p /backup/wal/%f'
I had run this statement today:
WITH table2_only_names AS ( SELECT id , name FROM table2 ) UPDATE table1 SET table2_name = table2_only_names.name FROM table2_only_names WHERE table1.table2_id = table2_only_names.id;
CREATE TABLE public.table1 ( id BIGINT PRIMARY KEY NOT NULL DEFAULT nextval('table1_id_seq'::regclass), table2_id BIGINT, table3_id BIGINT, positive_count INTEGER NOT NULL DEFAULT 0, neutral_count INTEGER NOT NULL DEFAULT 0, negative_count INTEGER NOT NULL DEFAULT 0, is_blocked BOOLEAN NOT NULL DEFAULT false, blocker_id BIGINT, group BIGINT, created TIMESTAMP WITH TIME ZONE, modified TIMESTAMP WITH TIME ZONE, table4_id BIGINT, name CHARACTER VARYING(255) ); CREATE UNIQUE INDEX idx1 ON table1 USING BTREE (table3_id, table2_id); CREATE INDEX idx2 ON table1 USING BTREE (table3_id); CREATE INDEX idx3 ON table1 USING BTREE (table2_id); CREATE INDEX idx4 ON table1 USING BTREE (group); CREATE INDEX idx5 ON table1 USING BTREE (blocker_id); CREATE INDEX idx6 ON table1 USING BTREE (table4_id);
15 Mio rows, table size ~3,4GB, index size 7GB
CREATE TABLE public.table2 ( id BIGINT PRIMARY KEY NOT NULL DEFAULT nextval('table2_id_seq'::regclass), name CHARACTER VARYING(255), );
10 Mio rows, table size ~2GB, index size 3,4GB
The runtime was about 55 minutes (not complaining here).
The amount of generated WAL files was unexpectedly huge. The aforementioned WAL archives are on a dedicated partition with has 100GB and at the point the query started, around 30GB were free.
It was not enough as after 15-20 minutes disk space was <10GB and I started to delete "older" WAL archives. I had to do this constantly up to the point were I was pretty sure I already had to delete WAL archive files generated by this very statement.
The tables in question were not used by any other process during that time but "normal" operations of other tables continued.
I've marked the time where I started the query and where it ended. You can clearly see where I deleted the WAL archive files :-)
To me, why so many are generated is a mystery and it currently is a problem because it's hardly foreseeable how much is needed and when operation strike which need one.
What am I missing to better understand how much space is needed? Are these things avoidable? Am I doing something wrong?