Why is count(x.*) slower than count(*)?

All we need is an easy explanation of the problem, so here it is.

explain select count(x.*) from customer x;
...
->  Partial Aggregate  (cost=27005.45..27005.46 rows=1 width=8)
  ->  Parallel Seq Scan on customer x  (cost=0.00..26412.56 rows=237156 width=994)

explain select count(*) from customer x;
...                                     

->  Partial Aggregate  (cost=27005.45..27005.46 rows=1 width=8)
  ->  Parallel Seq Scan on customer x  (cost=0.00..26412.56 rows=237156 width=0)

The COUNT(x.*) here makes the width in the explain result read unnecessary row data.

I thought they should be identical, but it seems not, why?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

Logically, both are identical – because x.* always counts, even when all columns are NULL.
But Postgres has a separate implementation for count(*).

It does not bother with any expression at all and only considers the existence of live rows. That’s slightly faster, which sums up to a relevant difference over many rows.
The performance penalty for count(x.*) grows with the number of columns / width of rows, and will be rather substantial for wide rows like yours (width=994).

It’s even documented explicitly:

count ( * ) → bigint

Computes the number of input rows.


count ( "any" ) → bigint

Computes the number of input rows in which the input value is not
null.

The gist of it: whenever you don’t care whether an expression is NULL, use count(*) instead.

Related:

Some other RDBMS do not have the same fast path for count(*). OTOH, counting all rows in a table is comparatively slow in Postgres due to its MVCC model that forces checking row visibility. See:

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

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