Re: ALTER TABLE ADD COLUMN fast default

Thanks for making those updates
.
On 20 February 2018 at 19:33, Andrew Dunstan
<andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
> On Mon, Feb 19, 2018 at 1:18 PM, David Rowley
> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> Since the attnum order in the missing values appears to be well
>> defined in ascending order, you can also simplify the comparison logic
>> in equalTupleDescs(). The inner-most nested loop shouldn't be
>> required.
>
> Well, this comment in the existing code in equalTupleDescs() suggests
> that in fact we can't rely on the attrdef items being in attnum order:
>
> /*
> * We can't assume that the items are always read from the system
> * catalogs in the same order; so use the adnum field to identify
> * the matching item to compare.
> */

On closer look, perhaps the reason that comment claims the order is
not well defined is that the use of the index depends on the value of
criticalRelcachesBuilt in the following:

pg_attribute_scan = systable_beginscan(pg_attribute_desc,
AttributeRelidNumIndexId,
criticalRelcachesBuilt,
NULL,
2, skey);

Otherwise, I see no reason for them to be out of order, as if I grep
for instances of "->missing = " I only see the place where the missing
attr details are set in RelationBuildTupleDesc() and one other place
in CreateTupleDescCopyConstr(), where it's simply just copied via
memcpy().

Nevertheless, it would be interesting to see how much a bsearch in
getmissingattr would help Tomas' case. Though, perhaps you're happy
enough with the performance already.

I'll make another pass over the updated code soon to see if I can see
anything else.

--
David Rowley http://www.2ndQuadrant.com/
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