Re: temp table "on commit delete rows": transaction overhead

2010/3/30 Tom Lane <tgl@sss.pgh.pa.us>:

> I don't recall any operations at transaction start for such tables,
> but there may be some.
>
Both in СommitTransaction(void) and  PrepareTransaction(void) we can
see PreCommit_on_commit_actions() call;

Here PreCommit_on_commit_actions()
<CUT>
            case ONCOMMIT_DELETE_ROWS:
                oids_to_truncate = lappend_oid(oids_to_truncate, oc->relid);
                break;
<CUT>

As a my opinion, the same action taked place at transaction start and commit.

To truncate rows of any temp tables, both  changed and unchanged(!)
after transaction looks as little reinsurance. Well.
But why do the same action _before_ any transaction?