Re: WAL usage calculation patch - Mailing list pgsql-hackers

On Fri, Apr 3, 2020 at 9:02 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, Apr 3, 2020 at 8:55 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > I think now I got the reason.  Basically, both of these records are
> > storing the FPW, and FPW size can vary based on the hole size on the
> > page.  If hold size is smaller the image length will be more, the
> > image_len= BLCKSZ-hole_size.  So in subsequent records, the image size
> > is bigger.
> >
>
> This means if we always re-create the database or may be keep
> full_page_writes to off, then we should get consistent WAL usage data
> for all tests.

With new database, it is always the same.  But, with full-page write,
I could see one of the create index is writing extra wal and if we
change the older then the new create index at that place will write
extra wal.  I guess that could be due to a non-in place update in some
of the system tables.

postgres[58554]=# create extension pg_stat_statements;
CREATE EXTENSION
postgres[58554]=#
postgres[58554]=# create table t1(id integer);
CREATE TABLE
postgres[58554]=# insert into t1 select * from generate_series(1, 1000000);
INSERT 0 1000000
postgres[58554]=# select * from pg_stat_statements_reset() ;
 pg_stat_statements_reset
--------------------------

(1 row)

postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 0);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_0 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 1);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_1 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 2);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_2 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 3);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_3 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 4);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_4 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 5);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_5 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 6);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_6 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 7);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_7 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# alter table t1 set (parallel_workers = 8);
ALTER TABLE
postgres[58554]=# vacuum;checkpoint;
VACUUM
CHECKPOINT
postgres[58554]=# create index t1_idx_parallel_8 ON t1(id);
CREATE INDEX
postgres[58554]=#
postgres[58554]=# select query, calls, wal_bytes, wal_records,
wal_num_fpw from pg_stat_statements where query ilike '%create
index%';
                  query                   | calls | wal_bytes |
wal_records | wal_num_fpw
------------------------------------------+-------+-----------+-------------+-------------
 create index t1_idx_parallel_0 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_1 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_3 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_2 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_4 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_8 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_6 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_7 ON t1(id) |     1 |  20355953 |
2766 |        2745
 create index t1_idx_parallel_5 ON t1(id) |     1 |  20359585 |
2767 |        2745

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com