explain analyze full page image and wal size meaning

Question

wonder about WAL....

drop table big;
CREATE TABLE big(
    bid integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
    s char(1000)
) WITH (fillfactor = 10);
insert into big(s) values('foo');
checkpoint;
select pg_current_wal_lsn() ;
 explain( analyze, costs off, verbose, timing off, buffers,settings, wal,timing off, summary)
delete from big;
select pg_current_wal_lsn() ;

returns:

                      QUERY PLAN
------------------------------------------------------
 Delete on public.big (actual rows=0 loops=1)
   Buffers: shared hit=2 dirtied=1
   WAL: records=1 fpi=1 bytes=1119
   ->  Seq Scan on public.big (actual rows=1 loops=1)
         Output: ctid
         Buffers: shared hit=1 dirtied=1
 Settings: search_path = 'public'
 Query Identifier: 6906021239059634709
 Planning:
   Buffers: shared hit=2
 Planning Time: 0.155 ms
 Execution Time: 0.206 ms
(12 rows)

    Parameter    | Value
-----------------+-------
 wal_compression | off
-----------------+-------
 full_page_writes| on

page meta:

select * from page_header(get_raw_page('big',0));

    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
------------+----------+-------+-------+-------+---------+----------+---------+-----------
 1/D2FB7F90 |        0 |     0 |    28 |  7160 |    8192 |     8192 |       4 |         0
(1 row)

2 lsn position do subtract operation return 1160.

questions:

1. fpi = 1 means dumped 8192 Bytes to WAL?
2. What does WAL: records=1 fpi=1 bytes=1119 1119 bytes mean?

related: https://wiki.postgresql.org/wiki/Full_page_writes

The idea is to switch to "physical" logging for the first modification of each page after a checkpoint, which is a fancy way of saying that we dump the whole 8KB page into the transaction log instead of describing how to change it.

Answer 1

The INSERT was the first transaction since the last checkpoint that modified that 8kB page (since the table is new), so PostgreSQL wrote the whole page to disk. That'd the meaning of FPI: Full Page Image.

The page is 8kB in size, but it has a hole in the middle. That hole is not copied to WAL, so the actual number of bytes is smaller.