4.26This is why query-before-upsert may be wasteful

Bad pattern:

For every customer:

QUERY
 ↓
does it exist?
 ↓
UPDATE / CREATE

100 customers means potentially:

100 queries
+
100 writes

If you already have a reliable external ID:

UPSERT

can remove the existence-query round trip entirely.

That's performance and simpler logic.