Hi all. I’m new to the PostgreSQL code and the mailing list, but I’ve had a lot 
of experience with using ICU in a different database product. So while I’m not 
up to speed on the code yet, I can offer some insights on using ICU.

> On Aug 30, 2016, at 9:12 PM, Peter Eisentraut 
> <peter.eisentr...@2ndquadrant.com> wrote:
>> How stable are the UCU locales? Most importantly, does ICU offer any
>> way to "pin" a locale version, so we can say "we want de_DE as it was
>> in ICU 4.6" and get consistent behaviour when the user sets up a
>> replica on some other system with ICU 4.8? Even if the German
>> government has changed its mind (again) about some details of the
>> language and 4.8 knows about the changes but 4.4 doesn’t?

ICU explicitly does not provide stability in their locales and collations. We 
pushed them hard to provide this, but between changes to the CLDR data and 
changes to the ICU code it just wasn’t feasible for them to provide version to 
version stability.

What they do offer is a compile option when building ICU to version all their 
APIs. So instead of calling icu_foo() you’d call icu_foo46(). (Or something 
like this - it’s been a few years since I actually worked with the ICU code.) 
This ultimately allows you to load multiple versions of the ICU library into a 
single program and provide stability by calling the appropriate version of the 
library. (Unfortunately, the OS - at least my Linux box - only provides the 
generic version of ICU and not the version annotated APIs, which means a 
separate compile of ICU is needed.)

The catch with this is that it means you likely want to expose the version 
information. In another note it was suggested to use something like fr_FR%icu. 
If you want to pin it to a specific version of ICU, you’ll likely need 
something like fr_FR%icu46. (There’s nothing wrong with supporting fr_FR%icu to 
give users an easy way of saying “give me the latest and greatest”, but you’d 
probably want to harden it to a specific ICU version internally.)

> I forgot to mention this, but the patch adds a collversion column that
> stores the collation version (provided by ICU).  And then when you
> upgrade ICU to something incompatible you get
> +           if (numversion != collform->collversion)
> +               ereport(WARNING,
> +                       (errmsg("ICU collator version mismatch"),
> +                        errdetail("The database was created using
> version 0x%08X, the library provides version 0x%08X.",
> +                                  (uint32) collform->collversion,
> (uint32) numversion),
> +                        errhint("Rebuild affected indexes, or build
> PostgreSQL with the right version of ICU.")));
> So you still need to manage this carefully, but at least you have a
> chance to learn about it.

Indexes are the obvious place where collation comes into play, and are 
relatively easy to address. But consider all the places where string 
comparisons can be done. For example, check constraints and referential 
constraints can depend on string comparisons. If the collation rules change 
because of a new version of ICU, the database can become inconsistent and will 
need a lot more work than an index rebuild.

> Suggestions for refining this are welcome.
> -- 
> Peter Eisentraut              http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Doug Doole

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