Reserve memory for one partition at a time

[Alena0704]

Partitions are scanned one after another, but the memory check counted all of them at once, 100kB each. So a table with 200 partitions needed 20MB just to start a query, and with less the query failed:

create table p (id int, d date) distributed by (id)
partition by range (d)
(start ('2020-01-01'::date) end ('2020-07-19'::date)
every ('1 day'::interval));

set optimizer = off; -- ORCA scans all partitions with one node
set statement_mem = '2MB';
select count(*) from p;

ERROR: insufficient memory reserved for statement

Count only the largest partition instead of all of them.

Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change (fix or feature with breaking changes)
• Documentation update

Breaking Changes

Test Plan

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• This PR contains AI-assisted code generation
• Requested review from cloudberry committers

Additional Context

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