about metadata: in Postgrest - you tell it which schema to expose and that means that anything inside it needs "tracking" and exposing, want it exposed, put it in that schema. Relations between entities ... that can be queried from the database itself, no need to ask the user for it.

timing out - it works up to a point, if you for example set a hard limit to 1s then to an attacker it just means he needs to do a few parallel requests and he still kills your db.

what postgrest does for protection: a - no fancy joins, joins are only done by a thing we call "embedding" and those are joins between tables that FKs defined between them so they are fast, if you need fancy joins then the user has to define the view doing the join and expose it to the api.

b - no functions applied to the return columns, like you cant generate something like "select md5(bigtextcolumn) ..."

c - the only way to "kill" the database under postgrest is to use a filter on a column that has no index ( name like "hello"), out of the box postgrest does not have protection build in for that, but it's easy to put a proxy in front and whenever there is a url parameter called "name" you just reply with "you are not allowed to filter by this column"