default_toast_compression only determines the compression method to be used when column values are toasted. It does not decide whether values are toasted at all.
Notably, the TOAST mechanism applies to values both stored inline and out-of-line. It's "The Oversized-Attribute Storage Technique".
You want to disable compression completely, not just choose the preferred algorithm. Before going there: Is it really worth the possible complications? Let me cite the full quote from your reference page:
Disabling TOAST compression
To not compress data twice, you can disable PostgreSQL TOAST
compression by setting column storage to
EXTERNAL. But it does not
make much difference:
- LZ4 is extremely fast.
- Both LZ4 and ZSTD have special logic to skip incompressible (or already compressed) parts of data.
Bold emphasis mine. Consider using the (new in Postgres 14) LZ4 compression method and be done with it. See:
What you ask
You could set
STORAGE per data type. Example for type
ALTER TYPE varchar SET (STORAGE = external);
This is then copied to every table column with that type, that is created after the change. The manual:
Normally a copy of
pg_type.typstorage of this column's type. For
TOAST-able data types, this can be altered after column creation to
control storage policy.
You have to change existing columns additionally.
Only base types can be altered this way (see
pg_type.typtype in the manual). And it may introduce caveats. The manual on
SET ( property = value [, ... ] )
This form is only applicable to base types. It allows adjustment of a subset of the base-type properties that can be set in
CREATE TYPE. Specifically, these properties can be changed:
STORAGE can be set to
main (see Section 70.2 for more information about what these mean).
However, changing from
plain to another setting requires superuser
privilege (because it requires that the type's C functions all be
TOAST-ready), and changing to plain from another setting is not
allowed at all (since the type may already have TOASTed values present
in the database). Note that changing this option doesn't by itself
change any stored data, it just sets the default TOAST strategy to be
used for table columns created in the future. See
to change the TOAST strategy for existing table columns.
That said, as a superuser, you could change all base types in the system with this query:
SET typstorage = 'e'
WHERE typtype = 'b'
AND typstorage = 'm'; -- "instead of MAIN"
I never tried that myself, and would not go there, and would strongly advise against it unless you know exactly what you are doing. Messing with system catalogs can break your database cluster.
I would just go with
default_toast_compression = 'lz4'.
Maybe experiment with one or two selected data types with
ALTER TYPE ... SET (STORAGE = external); first ...