dbt: complex-expression fallback metric double-aggregates on round-trip

[christianeu-db]

Summary

On ossie-to-msi, an Ossie metric whose expression matches no recognized pattern is stored as a SIMPLE metric that applies agg=SUM to an expression that is itself already SUM(...). The emitted MSI therefore encodes a double aggregation, and a round-trip (Ossie->MSI->Ossie) renders it as SUM(SUM(CASE WHEN ...)): silently, with no ConverterIssue.

The README already documents that unrecognized expressions are "stored verbatim" as best-effort. That is fine on its own; the defect is that the verbatim expression is paired with agg=SUM without stripping its outer aggregate, unlike the recognized path.

Repro (real converters, HEAD 3c94d16 - EXECUTED)

Forward pass: recognized vs. fallback, same agg, different handling of the outer aggregate:

RECOGNIZED  SUM(orders.amount)                              -> expr='amount'                    agg=SUM
FALLBACK    SUM(CASE WHEN orders.status = 'paid'            -> expr='SUM(CASE WHEN ...           agg=SUM
            THEN amount ELSE 0 END)                                 THEN amount ELSE 0 END)'

Full round-trip of the fallback metric:

INPUT  Ossie expr:  SUM(CASE WHEN orders.status = 'paid' THEN amount ELSE 0 END)
MSI type_params.expr: SUM(CASE WHEN orders.status = 'paid' THEN amount ELSE 0 END)   (agg=SUM)
OUTPUT Ossie expr:  SUM(SUM(CASE WHEN orders.status = 'paid' THEN amount ELSE 0 END))
issues (both directions): []

No ConverterIssue is raised in either direction.