Обсуждение: [ADMIN] pg_dump making schema output consistent.

James Lawrence <jljatone@gmail.com> writes:
> unfortunately pg_dump has two problems for this usecase.
> ...
> 2) column ordering within a table is inconsistent. couple of our tables
> columns get reordered (in a consistent manner) depending on which developer
> generates the schema.

> The inconsistent columns is the big problem, I suspect it is somehow
> related to the collation order of the database.

No, certainly not.  Ordinarily PG will preserve column order faithfully.
The only case I know of where it will not is if you have an inheritance
relationship and the child table's columns are out of order with respect
to the parent, which generally requires having done some sort of after-
the-fact DDL on the parent.  For instance

create table parent (a int, b int, c int);

create table child (d int, e int) inherits (parent);

-- at this point child has columns a, b, c, d, e

alter table parent add column f int;

-- at this point child has columns a, b, c, d, e, f


If you dump at this point, what'll be emitted is

create table parent (a int, b int, c int, f int);

create table child (d int, e int) inherits (parent);

If you restore that, the child has columns a, b, c, f, d, e, in that
order.  It'd be possible for pg_dump to preserve the original ordering,
but historically it hasn't done so, and from the aesthetic standpoint
it's not clear that'd be better.  After all, if you now do

create table child2 (d int, e int) inherits (parent);

then child2 is going to have a, b, c, f, d, e in either database.

I don't have enough info to say exactly how this is manifesting
as your problem, but I think you'll find it's something like that.
        regards, tom lane


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James Lawrence <jljatone@gmail.com> writes:
> Your explaination is also a little confusing, just for clarity going to ask
> some explicit questions.

> 1) 'Ordinarily PG will preserve column order faithfully.' what is column
> order? do you just mean the order in which columns were added to a table?
> your example implies this.

Whatever the table's column order is, as shown by e.g. "SELECT * FROM tab".

> 2) what is the difference between 'PG' and pg_dump? (I'm reading it as PG =
> postgresql server process, pg_dump as the dump tool).

In this case I was using it to mean the whole system, server and pg_dump
both.

> given that I've correctly understood your email, it doesn't directly
> address the issue I'm pointing out with column order in the schema dump.

Well, then I'm confused.  But you certainly haven't given enough
information for anyone to do more than guess at what your issue is.

> Given the fact without any schema details you hit on the fact these tables
> happen to use table inheritance, I'm inclined to lean towards that being
> the root cause of the differences in the schema dumps between machines.

It seems likely to me that it's something like this type of difference
having bit you when these databases were set up, ie the column orders
are already different on your two source databases.  Perhaps one was
made by dump/restore from the other, and the original had already had
some order-affecting DDL done on it.
        regards, tom lane


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