Re: Extension Templates S03E11

Robert Haas <robertmhaas@gmail.com> writes:
> In more normal cases, however, the system can (and probably should)
> figure out what was intended by choosing the *shortest* path to get to
> the intended version.  For example, if someone ships 1.0, 1.0--1.1,
> 1.1, and 1.1--1.2, the system should choose to run 1.1 and then
> 1.1--1.2, not 1.0 and then 1.0--1.1 and then 1.1--1.2.  But that can
> be automatic: only if there are two paths of equal length (as in the
> example in the previous paragraph) do we need help from the user to
> figure out what to do.

Yeah.

> We should also consider the possibility of a user trying to
> deliberately install and older release.  For example, if the user has
> 1.0, 1.0--1.1, 1.1, 1.1--1.2, and 1.2--1.0 (a downgrade script) with
> default_full_version = 1.2, an attempt to install 1.0 should run just
> the 1.0 script, NOT 1.2 and then 1.2--1.0.

In what I did, if you want version 1.0 and we have a script --1.0.sql
around, then we just use that script, never kicking the path chooser.

The path chooser at CREATE EXTENSION time is only execised when we don't
have a direct script to support that specific version you're asking.

> Putting all that together, I'm inclined to suggest that what we really
> need is a LIST of version numbers, rather than just one.  If there one
> path to the version we're installing is shorter than any other, we
> choose that, period.  If there are multiple paths of equal length, we

That's what Jeff did propose, yes.

> break the tie by choosing which version number appears first in the
> aforementioned list.  If that still doesn't break the tie, either
> because none of the starting points are mentioned in that list or
> because there are multiple equal-length paths starting in the same
> place, we give up and emit an error.

Jeff also did mention about tiebreakers without entering into any level
of details.

We won't be able to just use default_version as the tiebreaker list
here, because of the following example:
 default_version = 1.2, 1.0
 create extension foo version '1.1';

With such a setup it would prefer 1.2--1.1 to 1.0--1.1, which doesn't
look like what we want. Instead, we want
 default_version = 1.2 create_from_version_candidates = 1.0
 create extension foo version '1.1';

Then the tie breaker is the 1.0 in "create_from_version_candidates" so
we would run foo--1.0.sql and then foo--1.0--1.1.sql.

Comments?

Baring objections, I'm going to prepare a new branch to support
developping that behavior against only file based extensions, and submit
a spin-off patch to the current CF entry.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support