Tablesize and array text[] in postgresql

Question

Ok, here is the question. I have some table which I create like

create table maths( id1 bytea , id2 bytea , comment text[] )

there's more columns but that's insignificant. After creation I insert there ~500k rows and modify them by several queries each of them contains

array_append(comments, 'Some comment about current computing step')

And after all is done I get tablesize approx 1.6Gb's

If then I run

ALTER TABLE maths ALTER COLUMN comments TYPE varchar[] USING comments::varchar[];

the tablesize drops significantly, to approx ~300Mb's

Why is this happening? And how can I get similar result without altering column type?

note: I've tried to set column to varchar[] type on creation, but after doing all queries it still becomes 1.6Gb's in size and converting to text[] reduces it to 300Mb's again.

Answer 1

The ALTER TABLE .. ALTER TYPE .. does a full table rewrite, which compacts the table and indexes.

You'd get the same result from VACUUM FULL tablename; instead.

text and varchar have exactly identical storage.