BUG #15184: Planner overestimates number of rows in empty table

The following bug has been logged on the website:

Bug reference:      15184
Logged by:          Alexey Ermakov
Email address:      alexey.ermakov@dataegret.com
PostgreSQL version: 10.3
Operating system:   Linux
Description:

Hello,

in optimizer/util/plancat.c we have following lines
(https://github.com/postgres/postgres/blob/master/src/backend/optimizer/util/plancat.c#L957):

            if (curpages < 10 &&
                rel->rd_rel->relpages == 0 &&
                !rel->rd_rel->relhassubclass &&
                rel->rd_rel->relkind != RELKIND_INDEX)
                curpages = 10;

            /* report estimated # pages */
            *pages = curpages;
            /* quick exit if rel is clearly empty */
            if (curpages == 0)
            {
                *tuples = 0;
                *allvisfrac = 0;
                break;
            }

if table is really empty then in first condition we set curpages = 10 and
second condition doesn't apply.
so we estimate that empty table has 10 pages and 2550 rows (for table with
one int column)
which doesn't look good. is it intended behavior? perhaps we should add
(curpages > 0) condition ?

that overestimate could mess plans for example when we use temporary tables
which might be empty.

Thanks,
Alexey Ermakov