Re: TODO Item - Return compressed length of TOAST datatypes

Mark Kirkwood wrote:
> I did a few cleanups on the last patch. Please examine this one instead.
> The changes are:
>
> 1. Add documentation for pg_datum_length builtin.
> 2. Correct some typos in the code comments.
> 3. Move the code in toastfuncs.c to varlena.c as it is probably the
> correct place.
> 4. Use ereport instead of elog.
> 5 Quiet compiler warning in pg_datum_length.

I have modified your patch to simplify the logic, and renamed it to
pg_column_size(), to be consistent with our soon-to-be-added
pg_relation/tablespace/database functions from dbsize.

Here is a sample usage:

test=> CREATE TABLE test (x INT, y TEXT);
CREATE TABLE
test=> INSERT INTO test VALUES (4, repeat('x', 10000));
INSERT 0 1
test=> INSERT INTO test VALUES (4, repeat('x', 100000));
INSERT 0 1
test=> SELECT pg_column_size(x), pg_column_size(y) FROM test;
pg_column_size | pg_column_size
----------------+----------------
4 | 121
4 | 1152
(2 rows)

Interesting the 10-times larger column is 10-times larger in storage.
Do we have some limit on how many repeated values we can record?

Patch attached and applied.

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