From: pgsql-hackers-ow...@postgresql.org [pgsql-hackers-ow...@postgresql.org] 
On Behalf Of Tom Lane [t...@sss.pgh.pa.us]
Sent: Friday, February 11, 2011 7:35 PM
To: Dimitri Fontaine
Cc: David E. Wheeler; Robert Haas; Josh Berkus; pgsql-hackers@postgresql.org

"We will add logic to find a chain of update scripts leading from oldv to
v, in case that exact combination is not available in the extension's
script directory.  (NOTE: maybe in the CREATE ... FROM case, it would be
a better idea to not do that search, but insist on finding exactly
extname-oldv-v.sql?  That would provide at least a little bit of extra
protection against wrong FROM choice.  Not sure how much it helps

Version strings will have no hard-wired semantics except equality; we
don't need a sorting rule."

This has the side effect that you can also have downgrade scripts. I don't know 
if this is designed or just coincidental, so thought it would be worth 
mentioning. It can have some impact on how to find the update chain to the 
desired version (loops in the graph), although standard graph traversal 
algorithms should handle this just fine. The worst case is that if you are 
upgrading from 1.2 to 2.0 the path is 1.2 -> 1.1 -> 2.0, even if there exists a 
path 1.2 -> 1.8 -> 1.9 -> 2.0. This could potentially result in data loss, if 
the downgrade drops some columns or something like that.

All this can of course be avoided by documenting that even if it is possible to 
define downgrade script, don't do it...

 - Anssi
 PS. I hope this mail comes out somewhat sanely formatted, using our lovely 
OWA-webmail here...

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