Re: Need explanation on index size

[Guillaume Lelarge <guillaume@xxxxxxxxxxxx>]

Hi,

Le jeu. 24 sept. 2020 à 15:55, Guillaume Luchet <g.luchet@xxxxxxxxxxx> 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.