>>>>> "Ashutosh" == Ashutosh Sharma <ashu.coe...@gmail.com> writes:
Ashutosh> Hi All, Ashutosh> Today while trying to understand the code for ALTER TABLE in Ashutosh> PostgreSQL (basically the table rewrite part), I noticed that Ashutosh> we are switching to a per-tuple memory context even when Ashutosh> table rewrite is not required. For e.g.. consider the case Ashutosh> where we do ADD CONSTRAINTS (NOT NULL or CHECK) using ALTER Ashutosh> TABLE command. In this case, we just scan the tuple and Ashutosh> verify it for the given constraint instead of forming a new Ashutosh> tuple. So, not sure why do we switch to per-tuple memory Ashutosh> context when just adding the constraint. What makes you think that evaluating the constraint won't allocate memory? Switching contexts is virtually free (just an assignment to a global var). Resetting a context that's not been allocated in since its last reset is also virtually free (just checks a flag). In contrast, every single function (except special cases like index comparators) is free to allocate memory in its caller's context, for temporary use and for returning the result; how else would a condition like CHECK(substring(col for 3)='FOO') work, without allocating memory for the substring result? -- Andrew (irc:RhodiumToad)
