BUG #19102: Assertion failure in generate_orderedappend_paths with aggregate pushdown

The following bug has been logged on the website:

Bug reference:      19102
Logged by:          Kuntal Ghosh
Email address:      kuntalghosh.2007@gmail.com
PostgreSQL version: 18.0
Operating system:   aarch64 GNU/Linux
Description:

Hi,

The assertion "Assert(childrel->rows > 0)" in generate_orderedappend_paths()
fails when processing queries where aggregates are pushed down below merge
append. In such cases, childrel can be an upperrel (grouping rel) rather
than a base relation, and will have zero row-estimates.

Steps to reproduce the issue:

CREATE TABLE pagg_tab2 (id BIGINT, PRIMARY KEY (id)) PARTITION BY RANGE
(id);
CREATE TABLE pagg_tab2_0 PARTITION OF pagg_tab2 FOR VALUES FROM ('0') TO
('1000');
CREATE TABLE pagg_tab2_1 PARTITION OF pagg_tab2 FOR VALUES FROM ('1000') TO
('2000');

SET enable_partitionwise_aggregate = true;

EXPLAIN (COSTS OFF)  SELECT count(*)  FROM pagg_tab2 x  GROUP BY x.id  ORDER
BY x.id DESC  LIMIT 2;

Server crashes with assertion failure:
TRAP: FailedAssertion("childrel->rows > 0", File: "allpaths.c", Line: 1983)

To fix the issue, we can replace the direct division with
clamp_row_est(childrel->rows) to safely handle zero, and remove the
incorrect assertion:

-   Assert(childrel->rows > 0);
-   path_fraction /= childrel->rows;
+   path_fraction /= clamp_row_est(childrel->rows);

Thanks & regards,
Kuntal Ghosh