Question

When I have the following table:

CREATE TABLE test
(
  "id" integer NOT NULL,
  "myval" text NOT NULL,
  CONSTRAINT "test-id-pkey" PRIMARY KEY ("id")
)

When doing a lot of queries like the following:

UPDATE "test" set "myval" = "myval" || 'foobar' where "id" = 12345

Then the row myval will get larger and larger over time.
What will postgresql do? Where will it get the space from?

Can I avoid that postgresql needs more than one seek to read a particular myval-column?

Will postgresql do this automatically?

I know that normally I should try to normalize the data much more. But I need to read the value with one seek. Myval will enlarge by about 20 bytes with each update (that adds data). Some colums will have 1-2 updates, some 1000 updates.
Normally I would just use one new row instead of an update. But then selecting is getting slow.
So I came to the idea of denormalizing.

Answer 1

Change the FILLFACTOR of the table to create space for future updates. This can also be HOT updates because the text field doesn’t have an index, to make the update faster and autovacuum overhead lower because HOT updates use a microvacuum. The CREATE TABLE statement has some information about the FILLFACTOR.

ALTER TABLE test SET (fillfactor = 70);
-- do a table rebuild to blow some space in your current table:
VACUUM FULL ANALYZE test;
-- start testing

The value 70 is not the perfect setting, it depends on your unique situation. Maybe you’re fine with 90, it could also be 40 or something else.