Re: ALTER TYPE 2: skip already-provable no-work rewrites

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: ALTER TYPE 2: skip already-provable no-work rewrites
Date: 2011-02-14 18:12:21
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I'm even less familiar w/ this code than Robert, but figured I'd give a
shot at reviewing this anyway. I definitely like avoiding table
rewrites if I can get away with it. :)

First question is- why do you use #ifdef USE_ASSERT_CHECKING ..?
assert_enabled exists and will work the way you expect regardless, no?
Strikes me as unlikely that the checks would be a real performance

In ATColumnChangeSetWorkLevel(), I'm really not a huge fan of using a
passed-in argument to move through a list with.. I'd suggest using a
local variable that is set from what's passed in. I do see that's
inheirited, but still, you've pretty much redefined that entire
function anyway..

Also, I feel like that while(!tab->rewrite) really deserves more
explanation of what's happening than the function-level comment above
gives it. I'd prefer to see a comment above the while() explaining
that we're moving through a list to see if there's any level at which
expr is something complicated or is referring to a domain which has
constraints on it (presuming that I've followed what's going on
correctly, that is..). It also seems like it'd make more sense to me to
be a while() controlled by (IsA(expr, Var) && ((Var *) expr)->varattno
== varattno), since that's really the normal "stopping point".

These are all more stylistic issues than anything else. Last, but not
least, I do worry about how this may impact contrib modules, external
projects, or user-added data types, such as PostGIS, hstore, and ip4r.
Could we/should we limit this to only PG data types that we 'know' are
binary compatible? Is there any way or reason such external modules
could be fouled up by this?



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