A query planner will consider the idiosyncratic properties of your data to determine the most efficient way to execute your query, whereas a compiler is general

A query planner will consider the idiosyncratic properties of your data to determine the most efficient way to execute your query, whereas a compiler is generally blind to the data your program will be processing. So if you execute the query "SELECT * FROM (a, b) WHERE a.foo = b.bar", if you have many rows in `a` but few rows in `b`, then it's much for efficient to scan `b` than `a`. The query planner will keep track of properties like this & come up with tricks to speed up execution. But in the sense that "everything is a compiler", yeah you could totally think of a query planner as a compiler that takes in your query's AST and a statistical description of your data and lowers it to a query plan.