ICU, locale and collation question

On Mon, 2023-05-08 at 16:35 +0200, Oscar Carlberg wrote:

We have a bunch of existing Postgres 10 clusters running on CentOS 7,
which have been initialized (initdb) with these collation options;

-E 'UTF-8'
--lc-collate=sv_SE.UTF-8
--lc-ctype=sv_SE.UTF-8
--lc_monetary=sv_SE.UTF-8
--lc-numeric=sv_SE.UTF-8
--lc-time=sv_SE.UTF-8
--lc-messages=en_US.UTF-8

And createdb were provided with these locale options when setting up
databases:
--lc-collate=sv_SE.UTF-8
--lc-ctype=sv_SE.UTF-8

We're upgrading the servers using logical replication, and would like to
take the opportunity to switch to ICU rather than relying on glibc, to
avoid future problems with index corruption if using physical
replication between servers with different versions of glibc.

We're trying to figure out the most correct way to configure postgres to
achieve this. Currently we have:

-E 'UTF-8'
--locale-provider=icu
--icu-locale=sv-SE-x-icu

And createdb are provided with locale options:
--lc-collate=C
--lc-ctype=C

Is this a safe configuration to avoid index corruption, and other
problems, while still being compatible with the previous locale
settings?

Yes, that is safe.

But it is not compatible with the old setup when it comes to lc_time,
lc_messages and the others. You should use sv_SE.UTF-8 for these
locale categories.

Note that that won't cause problems with upgrading the C library.

Yours,
Laurenz Albe

On 5/8/23 22:07, Laurenz Albe wrote:

Yes, that is safe.

But it is not compatible with the old setup when it comes to lc_time,
lc_messages and the others. You should use sv_SE.UTF-8 for these
locale categories.

Note that that won't cause problems with upgrading the C library.

Yours,
Laurenz Albe

Thank you very much for clarifying!

Our initdb setup would then look like this for compatibility;
-E 'UTF-8'
--locale-provider=icu
--icu-locale=sv-SE-x-icu
--lc_monetary=sv_SE.UTF-8
--lc-numeric=sv_SE.UTF-8
--lc-time=sv_SE.UTF-8
--lc-messages=en_US.UTF-8

Should we still provide createdb with --lc-collate=C and --lc-ctype=C,
or should we set those to sv_SE.UTF-8 as well?

Best regards,
Oscar

--
Innehållet i detta e-postmeddelande är konfidentiellt och avsett endast för
adressaten.Varje spridning, kopiering eller utnyttjande av innehållet är
förbjuden utan tillåtelse av avsändaren. Om detta meddelande av misstag
gått till fel adressat vänligen radera det ursprungliga meddelandet och
underrätta avsändaren via e-post

On Tue, 2023-05-09 at 08:54 +0200, Oscar Carlberg wrote:

Should we still provide createdb with --lc-collate=C and --lc-ctype=C,
or should we set those to sv_SE.UTF-8 as well?

That depends on whether you want Swedish or binary sort order
(is "a" < "Z" or not?). The C collation is not good for natural
language sorting, but it is fast and not subject to collation changes.

Yours,
Laurenz Albe

On 09.05.23 08:54, Oscar Carlberg wrote:

Our initdb setup would then look like this for compatibility;
-E 'UTF-8'
--locale-provider=icu
--icu-locale=sv-SE-x-icu
--lc_monetary=sv_SE.UTF-8
--lc-numeric=sv_SE.UTF-8
--lc-time=sv_SE.UTF-8
--lc-messages=en_US.UTF-8

Should we still provide createdb with --lc-collate=C and --lc-ctype=C,
or should we set those to sv_SE.UTF-8 as well?

You should set those to something other than C. It doesn't matter much
what exactly, so what you have there is fine.

Setting it to C would for example affect the ability of the text search
functionality to detect words containing non-ASCII characters.

On Tue, May 9, 2023 at 11:24 AM Peter Eisentraut <
peter.eisentraut@enterprisedb.com> wrote:

On 09.05.23 08:54, Oscar Carlberg wrote:

Our initdb setup would then look like this for compatibility;
-E 'UTF-8'
--locale-provider=icu
--icu-locale=sv-SE-x-icu
--lc_monetary=sv_SE.UTF-8
--lc-numeric=sv_SE.UTF-8
--lc-time=sv_SE.UTF-8
--lc-messages=en_US.UTF-8

Should we still provide createdb with --lc-collate=C and --lc-ctype=C,
or should we set those to sv_SE.UTF-8 as well?

You should set those to something other than C. It doesn't matter much
what exactly, so what you have there is fine.

Setting it to C would for example affect the ability of the text search
functionality to detect words containing non-ASCII characters.

Doesn't searching LIKE 'abc%' behave much better for C than others. This

was the driving force for choosing C for us.
[EXPLAIN made it clear that it was range bound until 'abd']

On 10.05.23 07:02, Kirk Wolak wrote:

On Tue, May 9, 2023 at 11:24 AM Peter Eisentraut
<peter.eisentraut@enterprisedb.com
<mailto:peter.eisentraut@enterprisedb.com>> wrote:

On 09.05.23 08:54, Oscar Carlberg wrote:

Our initdb setup would then look like this for compatibility;
-E 'UTF-8'
--locale-provider=icu
--icu-locale=sv-SE-x-icu
--lc_monetary=sv_SE.UTF-8
--lc-numeric=sv_SE.UTF-8
--lc-time=sv_SE.UTF-8
--lc-messages=en_US.UTF-8

Should we still provide createdb with --lc-collate=C and

--lc-ctype=C,

or should we set those to sv_SE.UTF-8 as well?

You should set those to something other than C.  It doesn't matter much
what exactly, so what you have there is fine.

Setting it to C would for example affect the ability of the text search
functionality to detect words containing non-ASCII characters.

Doesn't searching LIKE 'abc%'  behave much better for C than others.
This was the driving force for choosing C for us.
[EXPLAIN made it clear that it was range bound until 'abd']

For that use, I would recommend making an index specifically on the
tables you need, instead of switching the whole database.

Also, if you are using the ICU provider for the database, then setting
lc_collation=C wouldn't even affect LIKE optimization, because the ICU
locale would be used.