Tom Lane wrote:
Robert Haas <robertmh...@gmail.com> writes:
Since the output in the previous email apparently wasn't sufficient
for you to understand what the problem is, here it is in more detail.
...
Adding a column to the toplevel parent of the inheritance hierarchy
and then dropping it again shouldn't leave a leftover copy of the
column in the grandchild.
Actually, it probably should. The inheritance rules were intentionally
designed to avoid dropping inherited columns that could conceivably
still contain valuable data. There isn't enough information in the
inhcount/islocal data structure to recognize that a multiply-inherited
column is ultimately derived from only one distant ancestor, but it was
agreed that an exact tracking scheme would be more complication than it
was worth. Instead, the mechanism is designed to ensure that no column
will be dropped if it conceivably is still wanted --- not that columns
might not be left behind and require another drop step.
This is not about dropping columns or not, but about proper maintenance
of the housekeeping of coninhcount, defined as "The number of direct
inheritance ancestors this constraint has. A constraint with a nonzero
number of ancestors cannot be dropped nor renamed".
Regard the following lattice (direction from top to bottom):
1
|\
2 3
\|\
4 5
\|
6
When adding a constraint to 1, the proper coninhcount for that
constraint on relation 6 is 2. But the code currently counts to 3, since
6 is reached by paths 1-2-4-5, 1-3-4-6, 1-3-5-6.
This wrong number is a bug.
*Please* go re-read the old discussions before you propose tampering
with this behavior. In particular I really really do not believe that
any one-line fix is going to make things better --- almost certainly
it will break other cases.
Our (more than one line) patch was explicitly designed to walk from a
specific parent to a child exactly once. It passes regression tests.
regards,
Yeb Havinga
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