Re: reduce null bitmap size

hi,

> yamt(at)mwd(dot)biglobe(dot)ne(dot)jp (YAMAMOTO Takashi) writes:
>> how about making heap_form_tuple and its variants use smaller natts for
>> tuples whose trailing columns are NULL?
>
> This idea has been proposed before, and rejected on the basis that it's
> unlikely to save enough to be worth the cycles needed to check for the
> case. Keep in mind that you aren't going to save anything at all unless
> the bitmap length decreases across a MAXALIGN boundary.

but it also means that, if one-bit descrease happens to cross the boundary,
you can save a MAXALIGN, right?

btw, while playing with this, i noticed that pg_column_size+rowtype+aggregate
produces unstable results. i guess it's because when nodeAgg handles
grp_firstTuple, the tuple is converted to a short varlena. i don't know
if it can be a real problem than just a weird looking.

YAMAMOTO Takashi

test=# create table t (i int);
CREATE TABLE
test=# insert into t values (1),(2),(3);
INSERT 0 3
test=# select pg_column_size(t.*) from t;
pg_column_size
----------------
28
28
28
(3 rows)

test=# select string_agg(pg_column_size(t.*)::text,',') from t;
string_agg
------------
25,28,28
(1 row)

test=#

>
> regards, tom lane
>
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