On Fri, Oct 21, 2016 at 7:15 PM, Serge Rielau <se...@rielau.com> wrote: > Some key design points requiring discussion: > 1. Storage of the “exist” (working name) default > Right now the patch stores the default value in its binary form as it > would be in the tuple into a BYTEA. > It would be feasible to store the pretty printed literal, however this > requires calling the io functions when a > tuple descriptor is built.
A pretty-printed literal is a terrible idea because input functions need not be immutable (e.g. timestamptz). I would have expected a pg_node_tree column containing a CONST node, but your bytea thing might be OK. > 2. The exist default is cached alongside the “current” default in the tuple > descriptor’s constraint structure. > Seems most natural too me, but debatable. No opinion (yet). > 3. Delayed vs. early expansion of the tuples. > To avoid having to decide when to copy tuple descriptors with or without > constraints and defaults > I have opted to expand the tuple at the core entry points. > How do I know I have them all? An omission means wrong results! Early expansion seems right. "How do I know I have them all?" - and not only all of the current ones but all future ones - seems like a core sticking point for this patch. > 4. attisnull() > This routine is used in many places, but to give correct result sit must > now be accompanied > by the tuple descriptor. This becomes moderately messy and it’s not > always clear where to get that. > Interestingly most usages are related to catalog lookups. > Assuming we have no intention to support fast default for catalog tables > we could keep using the > existing attisnull() api for catalog lookups and use a new version (name > tbd) for user tables. attisnull is not a thing. There's heap_attisnull and slot_attisnull. > 5. My head hurts looking at the PK/FK code - it’s not always clear which > tuple descriptor belongs > to which tuple I suggest ibuprofen and a stiff upper lip. > 6. Performance of the expansion code. > The current code needs to walk all defaults and then start padding by > filling in values. > But the outcome is always the same. We will produce the same payload and > the name null map. > It would be feasible to cache an “all defaults tuple”, remember the > offsets (and VARWIDTH, HASNULL) > for each attribute and then simply splice the short and default tuples > together. > This ought to be faster, but the meta data to cache is not insignificant > and the expansion code is messy enough > without this already. You could experiment to figure out if it makes any difference. > 7. Grooming > Obviously we can remove all exist defaults for a table from pg_attribute > whenever the table is rewrittem. > That’s easy. But might cause the table to expand a lot, which would suck. > But could we/should we keep track of the short tuples and either > eliminate them or drop exist defaults once they > become obsolete because there is no tuple short enough for them to > matter. I wouldn't mess with it. > 8. Do we need to worry about toasted defaults? Presumably. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers