Table with smaller data types seems to take more space on disk?



  • I have these two identical tables:

                      Table "public.region"
       Column    |   Type   | Collation | Nullable | Default 
    -------------+----------+-----------+----------+---------
     r_regionkey | integer      |           | not null | 
     r_name      | char(25)     |           |          | 
     r_comment   | char(152)     |           |          | 
    Indexes:
        "region_pkey" PRIMARY KEY, btree (r_regionkey)
    

    and

                      Table "public.region2"
       Column    |   Type   | Collation | Nullable | Default 
    -------------+----------+-----------+----------+---------
     r_regionkey | smallint |           | not null | 
     r_name      | text     |           |          | 
     r_comment   | text     |           |          | 
    Indexes:
        "region_pkey" PRIMARY KEY, btree (r_regionkey)
    

    I am using smallint and text in order to save space, but weirdly this is the result:

    select pg_size_pretty(pg_table_size('region'))
    

    returns 8192 bytes while

    select pg_size_pretty(pg_table_size('region2'))
    

    returns 48 kB.

    Why is region2 taking more space, even though I am using smallint instead of integer and text instead of char(n)?



  • After running VACUUM FULL public.region; and VACUUM FULL public.region2;, test again with:

    SELECT pg_relation_size('public.region');
    

    Three possible issues:

    1. The obvious reason: table bloat from updates or deletes. Removed by VACUUM FULL.

    2. Schema-qualified names ('public.region' instead of just 'region') makes sure you don't measure the wrong table in a different schema by accident. Probably not the case.

    3. pg_table_size() includes auxiliary relation forks (files), which may be filled for one table, but not for the other. For your purpose, the more accurate test is with pg_relation_size().

    https://www.postgresql.org/docs/current/functions-admin.html#FUNCTIONS-ADMIN-DBSIZE

    pg_table_size ( regclass ) → bigint

    Computes the disk space used by the specified table, excluding indexes (but including its TOAST table if any, free space map, and visibility map).

    And:

    pg_relation_size ( relation regclass [, fork text ] ) → bigint

    Computes the disk space used by one “fork” of the specified relation. (Note that for most purposes it is more convenient to use the higher-level functions pg_total_relation_size or pg_table_size, which sum the sizes of all forks.) With one argument, this returns the size of the main data fork of the relation. The second argument can be provided to specify which fork to examine:

    • main returns the size of the main data fork of the relation.

    • fsm returns the size of the Free Space Map (see https://www.postgresql.org/docs/current/storage-vm.html ) associated with the relation.

    • vm returns the size of the Visibility Map (see https://www.postgresql.org/docs/current/storage-vm.html ) associated with the relation.

    • init returns the size of the initialization fork, if any, associated with the relation.

    Since your example has no https://www.postgresql.org/docs/current/storage-toast.html , only https://www.postgresql.org/docs/current/storage-fsm.html and https://www.postgresql.org/docs/current/storage-vm.html make a difference. Those are typically negligible in size for bigger tables, but relevant for your minimal test. Both may go down to "0 bytes" after VACUUM FULL.

    Test with more rows (thousands).

    Related:

    • https://dba.stackexchange.com/questions/23879/measure-the-size-of-a-postgresql-table-row/23933#23933



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