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Max() over a text column is a sort, and the engines sort text differently:
MariaDB's utf8mb4 collations fold case, the CI PostgreSQL orders by byte
value. MAX('abc','ABD') is 'ABD' on MariaDB and 'abc' on PostgreSQL --
confirmed on CI in the probe attached to #56241.
The parity effort wrapped many descriptive columns in Max() on the reasoning
that it returns the value MySQL picked arbitrarily. Where the column is
functionally dependent on the group key that holds and the wrap is a genuine
no-op. Where it genuinely varies -- description, item_name, uom and their
warehouses all describe a LINE, not the item -- it does not: MySQL picked a
row, not a maximum, and the sort now diverges between engines. Aggregating
each column separately can also pair one line's description with another's
warehouse, or a uom with the wrong conversion factor.
Take those columns from a single real line instead, the first by idx.
Only groups built from more than one line need it. Each query now also selects
Count(<line>.name).distinct(), and the representative pass returns immediately
when no group has more than one line -- in that case Max() of a single value
is already exact and collation cannot apply. A BOM with no repeated item
therefore issues no extra query at all, which matters because the explosion
and sub-assembly resolution recurse per sub-BOM. Genuine repeats are memoised
per request.
Sites covered: BOM explosion and sub-item queries, sub-assembly raw materials,
get_bom_items_as_dict, BOM Stock Analysis (both queries), Requested Items to
Order and Receive, Pending SO Items for Purchase Request, and Job Card
secondary items.