affect vs effect

"affected rows" makes it an adjective, right?

Essentially, yes. Verbs can become adjectives by turning into participles. (Sorry you asked yet?)
Participles are verb forms ending in ‑ing (present participles) or -ed or -en (past participles) that modify nouns.

While effect is usually a noun, it can be a verb when used in the sense of to bring about e.g. "The update method is used to effect change on the rows." However, once the method has run we would want to know which (or how many) rows were affected not effected.

So, for me, the correct DB function name is affected_rows.

In the interests of full disclosure, there are cases where a noun can act as an adjective e.g. "ticket office".