My impression was the parent poster was talking about order of operations like projection and selection, where you might more commonly write: Π(σ(R)) instead o

My impression was the parent poster was talking about order of operations like projection and selection, where you might more commonly write: Π(σ(R)) instead of σ(Π(R)) and not about whether relational algebra uses prefix or postfix notation: Π(σ(R)) vs. R > σ > Π SQL's WHERE statement (and others) works totally differently from SELECT in that regard, so it doesn't make much sense to say that "SELECT comes first because relational algebra".