While looking at the compressibility of WAL files generated by pgbench, which is close to 90%, I first thought its because of the "filler" column in the accounts table. But a comment in pgbench.c says:
/*
* Note: TPC-B requires at least 100 bytes per row, and the "filler"
* fields in these table declarations were intended to comply with that.
* But because they default to NULLs, they don't actually take any space.
* We could fix that by giving them non-null default values. However, that
* would completely break comparability of pgbench results with prior
* versions. Since pgbench has never pretended to be fully TPC-B
* compliant anyway, we stick with the historical behavior.
*/
The comment about them being NULL and hence not taking up any space is added by commit b7a67c2840f193f in response to this bug report:
On a more careful look, it seems the original bug report complained about all tables except accounts. And all other tables indeed have "filler" as NULL. But the way comment is written it seems as if it applies to all DDLs. Should we just fix the comment and say its applicable for all tables except accounts ?