Re: Need explanation on index size

Hi,

Le jeu. 24 sept. 2020 à 15:55, Guillaume Luchet <g(dot)luchet(at)bilendi(dot)com> a
écrit :

> Hi,
>
> I’m facing of a comportement I don’t understand on indexes, here a quick
> example to reproduce my problem
>
>
> test=# select version();
> version
>
>
> ----------------------------------------------------------------------------------------------------------------------------------
> PostgreSQL 12.4 (Debian 12.4-1.pgdg90+1) on x86_64-pc-linux-gnu, compiled
> by gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, 64-bit
> (1 row)
>
> test=# create table plop (col_a int, col_b int, col_c int);
> CREATE TABLE
>
> test=# create unique index on plop (col_a);
> CREATE INDEX
>
> test=# create index on plop(col_b);
> CREATE INDEX
>
> test=# insert into plop (col_a, col_b) select generate_series(1, 10000),
> generate_series(1, 10000);
> INSERT 0 10000
>
> test=# SELECT schemaname, tablename,
> pg_size_pretty(SIZE) AS size_pretty,
> pg_size_pretty(total_size) AS total_size_pretty
> FROM (SELECT *, pg_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS SIZE,
> pg_total_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS total_size
> FROM pg_tables) AS TABLES where tablename = 'plop';
> schemaname | tablename | size_pretty | total_size_pretty
> ------------+-----------+-------------+-------------------
> public | plop | 360 kB | 864 kB
> (1 row)
>
> test=# update plop set col_c = floor(random() * 10 + 1)::int;
> UPDATE 10000
>
> test=# SELECT schemaname, tablename,
> pg_size_pretty(SIZE) AS size_pretty,
> pg_size_pretty(total_size) AS total_size_pretty
> FROM (SELECT *, pg_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS SIZE,
> pg_total_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS total_size
> FROM pg_tables) AS TABLES where tablename = 'plop';
> schemaname | tablename | size_pretty | total_size_pretty
> ------------+-----------+-------------+-------------------
> public | plop | 792 kB | 2160 kB
> (1 row)
>
> test=# reindex table plop;
> REINDEX
>
> test=# SELECT schemaname, tablename,
> pg_size_pretty(SIZE) AS size_pretty,
> pg_size_pretty(total_size) AS total_size_pretty
> FROM (SELECT *, pg_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS SIZE,
> pg_total_relation_size(quote_ident(schemaname) || '.' ||
> quote_ident(tablename)) AS total_size
> FROM pg_tables) AS TABLES where tablename = 'plop';
> schemaname | tablename | size_pretty | total_size_pretty
> ------------+-----------+-------------+-------------------
> public | plop | 792 kB | 1304 kB
> (1 row)
>
> I don’t understand why after the update where I only update a non indexed
> column the indexes size is growing. Is it something someone can explain ?
>
>
Every tuple is now on a different location on the table (remember that
update in PostgreSQL is more something like delete+insert). So even if the
value of the column doesn't change, its tuple location changes, so the
index needs to be updated to reflect that change.

--
Guillaume.