On 14.02.23 17:53, Laurenz Albe wrote:
On Mon, 2023-02-06 at 22:16 +0100, Peter Eisentraut wrote:
Right.  Here is a new patch with this fixed.

Thanks.  I played some more with it, and still are still some missing
odds and ends:

- There is a new option ICU_RULES to CREATE DATABASE, but it is not
   reflected in \h CREATE DATABASE.  sql_help_CREATE_DATABASE() needs to
   be amended.

Fixed.

- There is no way to show the rules except by querying "pg_collation" or
   "pg_database".  I think it would be good to show the rules with
   \dO+ and \l+.

Fixed. I adjusted the order of the columns a bit, to make the overall picture more sensible. The locale provider column is now earlier, since it indicates which of the subsequent columns are applicable.

- If I create a collation "x" with RULES and then create a database
   with "ICU_LOCALE x", the rules are not copied over.

   I don't know if that is intended or not, but it surprises me.
   Should that be a WARNING?  Or, since creating a database with a collation
   that does not exist in "template0" doesn't make much sense (or does it?),
   is there a way to forbid that?

This is a misunderstanding of how things work. The value of the database ICU_LOCALE attribute is passed to the ICU library. It does not refer to a PostgreSQL collation object.
From d6ee2e92af9d1a25fe316e5c93d8aa20179658da Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <pe...@eisentraut.org>
Date: Mon, 20 Feb 2023 09:46:48 +0100
Subject: [PATCH v5] Allow tailoring of ICU locales with custom rules

This exposes the ICU facility to add custom collation rules to a
standard collation.

Discussion: 
https://www.postgresql.org/message-id/flat/821c71a4-6ef0-d366-9acf-bb8e367f7...@enterprisedb.com
---
 doc/src/sgml/catalogs.sgml                    |  18 ++++
 doc/src/sgml/ref/create_collation.sgml        |  22 ++++
 doc/src/sgml/ref/create_database.sgml         |  13 +++
 src/backend/catalog/pg_collation.c            |   5 +
 src/backend/commands/collationcmds.c          |  23 +++-
 src/backend/commands/dbcommands.c             |  51 ++++++++-
 src/backend/utils/adt/pg_locale.c             |  41 ++++++-
 src/backend/utils/init/postinit.c             |  11 +-
 src/bin/psql/describe.c                       | 100 +++++++++++-------
 src/include/catalog/pg_collation.h            |   2 +
 src/include/catalog/pg_database.h             |   3 +
 src/include/utils/pg_locale.h                 |   1 +
 .../regress/expected/collate.icu.utf8.out     |  30 ++++++
 src/test/regress/expected/psql.out            |  18 ++--
 src/test/regress/sql/collate.icu.utf8.sql     |  13 +++
 15 files changed, 295 insertions(+), 56 deletions(-)

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index c1e4048054..746baf5053 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2428,6 +2428,15 @@ <title><structname>pg_collation</structname> 
Columns</title>
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>collicurules</structfield> <type>text</type>
+      </para>
+      <para>
+       ICU collation rules for this collation object
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>collversion</structfield> <type>text</type>
@@ -3106,6 +3115,15 @@ <title><structname>pg_database</structname> 
Columns</title>
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>daticurules</structfield> <type>text</type>
+      </para>
+      <para>
+       ICU collation rules for this database
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>datcollversion</structfield> <type>text</type>
diff --git a/doc/src/sgml/ref/create_collation.sgml 
b/doc/src/sgml/ref/create_collation.sgml
index 136976165c..289f8147f1 100644
--- a/doc/src/sgml/ref/create_collation.sgml
+++ b/doc/src/sgml/ref/create_collation.sgml
@@ -27,6 +27,7 @@
     [ LC_CTYPE = <replaceable>lc_ctype</replaceable>, ]
     [ PROVIDER = <replaceable>provider</replaceable>, ]
     [ DETERMINISTIC = <replaceable>boolean</replaceable>, ]
+    [ RULES = <replaceable>rules</replaceable>, ]
     [ VERSION = <replaceable>version</replaceable> ]
 )
 CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM 
<replaceable>existing_collation</replaceable>
@@ -149,6 +150,19 @@ <title>Parameters</title>
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><replaceable>rules</replaceable></term>
+
+     <listitem>
+      <para>
+       Specifies additional collation rules to customize the behavior of the
+       collation.  This is supported for ICU only.  See <ulink
+       
url="https://unicode-org.github.io/icu/userguide/collation/customization/"/>
+       for details on the syntax.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><replaceable>version</replaceable></term>
 
@@ -228,6 +242,14 @@ <title>Examples</title>
 </programlisting>
   </para>
 
+  <para>
+   To create a collation using the ICU provider, based on the English ICU
+   locale, with custom rules:
+<programlisting>
+<![CDATA[CREATE COLLATION en_custom (provider = icu, locale = 'en', rules = 
'&a < g');]]>
+</programlisting>
+  </para>
+
   <para>
    To create a collation from an existing collation:
 <programlisting>
diff --git a/doc/src/sgml/ref/create_database.sgml 
b/doc/src/sgml/ref/create_database.sgml
index 57d13e34c2..6f62161b80 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/doc/src/sgml/ref/create_database.sgml
@@ -30,6 +30,7 @@
            [ LC_COLLATE [=] <replaceable 
class="parameter">lc_collate</replaceable> ]
            [ LC_CTYPE [=] <replaceable 
class="parameter">lc_ctype</replaceable> ]
            [ ICU_LOCALE [=] <replaceable 
class="parameter">icu_locale</replaceable> ]
+           [ ICU_RULES [=] <replaceable 
class="parameter">icu_rules</replaceable> ]
            [ LOCALE_PROVIDER [=] <replaceable 
class="parameter">locale_provider</replaceable> ]
            [ COLLATION_VERSION = <replaceable>collation_version</replaceable> ]
            [ TABLESPACE [=] <replaceable 
class="parameter">tablespace_name</replaceable> ]
@@ -192,6 +193,18 @@ <title>Parameters</title>
       </listitem>
      </varlistentry>
 
+     <varlistentry id="create-database-icu-rules">
+      <term><replaceable class="parameter">icu_rules</replaceable></term>
+      <listitem>
+       <para>
+        Specifies additional collation rules to customize the behavior of the
+        collation.  This is supported for ICU only.  See <ulink
+        
url="https://unicode-org.github.io/icu/userguide/collation/customization/"/>
+        for details on the syntax.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="create-database-locale-provider">
       <term><replaceable>locale_provider</replaceable></term>
 
diff --git a/src/backend/catalog/pg_collation.c 
b/src/backend/catalog/pg_collation.c
index 287b13725d..fd022e6fc2 100644
--- a/src/backend/catalog/pg_collation.c
+++ b/src/backend/catalog/pg_collation.c
@@ -50,6 +50,7 @@ CollationCreate(const char *collname, Oid collnamespace,
                                int32 collencoding,
                                const char *collcollate, const char *collctype,
                                const char *colliculocale,
+                               const char *collicurules,
                                const char *collversion,
                                bool if_not_exists,
                                bool quiet)
@@ -194,6 +195,10 @@ CollationCreate(const char *collname, Oid collnamespace,
                values[Anum_pg_collation_colliculocale - 1] = 
CStringGetTextDatum(colliculocale);
        else
                nulls[Anum_pg_collation_colliculocale - 1] = true;
+       if (collicurules)
+               values[Anum_pg_collation_collicurules - 1] = 
CStringGetTextDatum(collicurules);
+       else
+               nulls[Anum_pg_collation_collicurules - 1] = true;
        if (collversion)
                values[Anum_pg_collation_collversion - 1] = 
CStringGetTextDatum(collversion);
        else
diff --git a/src/backend/commands/collationcmds.c 
b/src/backend/commands/collationcmds.c
index eb62d285ea..c51e3afdb4 100644
--- a/src/backend/commands/collationcmds.c
+++ b/src/backend/commands/collationcmds.c
@@ -64,10 +64,12 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
        DefElem    *lcctypeEl = NULL;
        DefElem    *providerEl = NULL;
        DefElem    *deterministicEl = NULL;
+       DefElem    *rulesEl = NULL;
        DefElem    *versionEl = NULL;
        char       *collcollate;
        char       *collctype;
        char       *colliculocale;
+       char       *collicurules;
        bool            collisdeterministic;
        int                     collencoding;
        char            collprovider;
@@ -99,6 +101,8 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
                        defelp = &providerEl;
                else if (strcmp(defel->defname, "deterministic") == 0)
                        defelp = &deterministicEl;
+               else if (strcmp(defel->defname, "rules") == 0)
+                       defelp = &rulesEl;
                else if (strcmp(defel->defname, "version") == 0)
                        defelp = &versionEl;
                else
@@ -161,6 +165,12 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
                else
                        colliculocale = NULL;
 
+               datum = SysCacheGetAttr(COLLOID, tp, 
Anum_pg_collation_collicurules, &isnull);
+               if (!isnull)
+                       collicurules = TextDatumGetCString(datum);
+               else
+                       collicurules = NULL;
+
                ReleaseSysCache(tp);
 
                /*
@@ -182,6 +192,7 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
                collcollate = NULL;
                collctype = NULL;
                colliculocale = NULL;
+               collicurules = NULL;
 
                if (providerEl)
                        collproviderstr = defGetString(providerEl);
@@ -191,6 +202,9 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
                else
                        collisdeterministic = true;
 
+               if (rulesEl)
+                       collicurules = defGetString(rulesEl);
+
                if (versionEl)
                        collversion = defGetString(versionEl);
 
@@ -297,6 +311,7 @@ DefineCollation(ParseState *pstate, List *names, List 
*parameters, bool if_not_e
                                                         collcollate,
                                                         collctype,
                                                         colliculocale,
+                                                        collicurules,
                                                         collversion,
                                                         if_not_exists,
                                                         false);        /* not 
quiet */
@@ -680,7 +695,7 @@ create_collation_from_locale(const char *locale, int nspid,
         */
        collid = CollationCreate(locale, nspid, GetUserId(),
                                                         COLLPROVIDER_LIBC, 
true, enc,
-                                                        locale, locale, NULL,
+                                                        locale, locale, NULL, 
NULL,
                                                         
get_collation_actual_version(COLLPROVIDER_LIBC, locale),
                                                         true, true);
        if (OidIsValid(collid))
@@ -755,7 +770,7 @@ win32_read_locale(LPWSTR pStr, DWORD dwFlags, LPARAM lparam)
 
                collid = CollationCreate(alias, param->nspid, GetUserId(),
                                                                 
COLLPROVIDER_LIBC, true, enc,
-                                                                localebuf, 
localebuf, NULL,
+                                                                localebuf, 
localebuf, NULL, NULL,
                                                                 
get_collation_actual_version(COLLPROVIDER_LIBC, localebuf),
                                                                 true, true);
                if (OidIsValid(collid))
@@ -889,7 +904,7 @@ pg_import_system_collations(PG_FUNCTION_ARGS)
 
                        collid = CollationCreate(alias, nspid, GetUserId(),
                                                                         
COLLPROVIDER_LIBC, true, enc,
-                                                                        
locale, locale, NULL,
+                                                                        
locale, locale, NULL, NULL,
                                                                         
get_collation_actual_version(COLLPROVIDER_LIBC, locale),
                                                                         true, 
true);
                        if (OidIsValid(collid))
@@ -951,7 +966,7 @@ pg_import_system_collations(PG_FUNCTION_ARGS)
                        collid = CollationCreate(psprintf("%s-x-icu", langtag),
                                                                         nspid, 
GetUserId(),
                                                                         
COLLPROVIDER_ICU, true, -1,
-                                                                        NULL, 
NULL, iculocstr,
+                                                                        NULL, 
NULL, iculocstr, NULL,
                                                                         
get_collation_actual_version(COLLPROVIDER_ICU, iculocstr),
                                                                         true, 
true);
                        if (OidIsValid(collid))
diff --git a/src/backend/commands/dbcommands.c 
b/src/backend/commands/dbcommands.c
index ef05633bb0..e75efffa08 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -119,6 +119,7 @@ static bool get_db_info(const char *name, LOCKMODE lockmode,
                                                int *encodingP, bool 
*dbIsTemplateP, bool *dbAllowConnP,
                                                TransactionId *dbFrozenXidP, 
MultiXactId *dbMinMultiP,
                                                Oid *dbTablespace, char 
**dbCollate, char **dbCtype, char **dbIculocale,
+                                               char **dbIcurules,
                                                char *dbLocProvider,
                                                char **dbCollversion);
 static void remove_dbtablespaces(Oid db_id);
@@ -675,6 +676,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
        char       *src_collate = NULL;
        char       *src_ctype = NULL;
        char       *src_iculocale = NULL;
+       char       *src_icurules = NULL;
        char            src_locprovider = '\0';
        char       *src_collversion = NULL;
        bool            src_istemplate;
@@ -698,6 +700,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
        DefElem    *dcollate = NULL;
        DefElem    *dctype = NULL;
        DefElem    *diculocale = NULL;
+       DefElem    *dicurules = NULL;
        DefElem    *dlocprovider = NULL;
        DefElem    *distemplate = NULL;
        DefElem    *dallowconnections = NULL;
@@ -710,6 +713,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
        char       *dbcollate = NULL;
        char       *dbctype = NULL;
        char       *dbiculocale = NULL;
+       char       *dbicurules = NULL;
        char            dblocprovider = '\0';
        char       *canonname;
        int                     encoding = -1;
@@ -775,6 +779,12 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                                errorConflictingDefElem(defel, pstate);
                        diculocale = defel;
                }
+               else if (strcmp(defel->defname, "icu_rules") == 0)
+               {
+                       if (dicurules)
+                               errorConflictingDefElem(defel, pstate);
+                       dicurules = defel;
+               }
                else if (strcmp(defel->defname, "locale_provider") == 0)
                {
                        if (dlocprovider)
@@ -893,6 +903,8 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                dbctype = defGetString(dctype);
        if (diculocale && diculocale->arg)
                dbiculocale = defGetString(diculocale);
+       if (dicurules && dicurules->arg)
+               dbicurules = defGetString(dicurules);
        if (dlocprovider && dlocprovider->arg)
        {
                char       *locproviderstr = defGetString(dlocprovider);
@@ -958,7 +970,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                                         &src_dboid, &src_owner, &src_encoding,
                                         &src_istemplate, &src_allowconn,
                                         &src_frozenxid, &src_minmxid, 
&src_deftablespace,
-                                        &src_collate, &src_ctype, 
&src_iculocale, &src_locprovider,
+                                        &src_collate, &src_ctype, 
&src_iculocale, &src_icurules, &src_locprovider,
                                         &src_collversion))
                ereport(ERROR,
                                (errcode(ERRCODE_UNDEFINED_DATABASE),
@@ -1006,6 +1018,8 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                dblocprovider = src_locprovider;
        if (dbiculocale == NULL && dblocprovider == COLLPROVIDER_ICU)
                dbiculocale = src_iculocale;
+       if (dbicurules == NULL && dblocprovider == COLLPROVIDER_ICU)
+               dbicurules = src_icurules;
 
        /* Some encodings are client only */
        if (!PG_VALID_BE_ENCODING(encoding))
@@ -1097,6 +1111,9 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 
                if (dblocprovider == COLLPROVIDER_ICU)
                {
+                       char       *val1;
+                       char       *val2;
+
                        Assert(dbiculocale);
                        Assert(src_iculocale);
                        if (strcmp(dbiculocale, src_iculocale) != 0)
@@ -1105,6 +1122,19 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                                                 errmsg("new ICU locale (%s) is 
incompatible with the ICU locale of the template database (%s)",
                                                                dbiculocale, 
src_iculocale),
                                                 errhint("Use the same ICU 
locale as in the template database, or use template0 as template.")));
+
+                       val1 = dbicurules;
+                       if (!val1)
+                               val1 = "";
+                       val2 = src_icurules;
+                       if (!val2)
+                               val2 = "";
+                       if (strcmp(val1, val2) != 0)
+                               ereport(ERROR,
+                                               
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+                                                errmsg("new ICU collation 
rules (%s) are incompatible with the ICU collation rules of the template 
database (%s)",
+                                                               val1, val2),
+                                                errhint("Use the same ICU 
collation rules as in the template database, or use template0 as template.")));
                }
        }
 
@@ -1313,6 +1343,10 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
                new_record[Anum_pg_database_daticulocale - 1] = 
CStringGetTextDatum(dbiculocale);
        else
                new_record_nulls[Anum_pg_database_daticulocale - 1] = true;
+       if (dbicurules)
+               new_record[Anum_pg_database_daticurules - 1] = 
CStringGetTextDatum(dbicurules);
+       else
+               new_record_nulls[Anum_pg_database_daticurules - 1] = true;
        if (dbcollversion)
                new_record[Anum_pg_database_datcollversion - 1] = 
CStringGetTextDatum(dbcollversion);
        else
@@ -1526,7 +1560,7 @@ dropdb(const char *dbname, bool missing_ok, bool force)
        pgdbrel = table_open(DatabaseRelationId, RowExclusiveLock);
 
        if (!get_db_info(dbname, AccessExclusiveLock, &db_id, NULL, NULL,
-                                        &db_istemplate, NULL, NULL, NULL, 
NULL, NULL, NULL, NULL, NULL, NULL))
+                                        &db_istemplate, NULL, NULL, NULL, 
NULL, NULL, NULL, NULL, NULL, NULL, NULL))
        {
                if (!missing_ok)
                {
@@ -1726,7 +1760,7 @@ RenameDatabase(const char *oldname, const char *newname)
        rel = table_open(DatabaseRelationId, RowExclusiveLock);
 
        if (!get_db_info(oldname, AccessExclusiveLock, &db_id, NULL, NULL,
-                                        NULL, NULL, NULL, NULL, NULL, NULL, 
NULL, NULL, NULL, NULL))
+                                        NULL, NULL, NULL, NULL, NULL, NULL, 
NULL, NULL, NULL, NULL, NULL))
                ereport(ERROR,
                                (errcode(ERRCODE_UNDEFINED_DATABASE),
                                 errmsg("database \"%s\" does not exist", 
oldname)));
@@ -1836,7 +1870,7 @@ movedb(const char *dbname, const char *tblspcname)
        pgdbrel = table_open(DatabaseRelationId, RowExclusiveLock);
 
        if (!get_db_info(dbname, AccessExclusiveLock, &db_id, NULL, NULL,
-                                        NULL, NULL, NULL, NULL, 
&src_tblspcoid, NULL, NULL, NULL, NULL, NULL))
+                                        NULL, NULL, NULL, NULL, 
&src_tblspcoid, NULL, NULL, NULL, NULL, NULL, NULL))
                ereport(ERROR,
                                (errcode(ERRCODE_UNDEFINED_DATABASE),
                                 errmsg("database \"%s\" does not exist", 
dbname)));
@@ -2599,6 +2633,7 @@ get_db_info(const char *name, LOCKMODE lockmode,
                        int *encodingP, bool *dbIsTemplateP, bool *dbAllowConnP,
                        TransactionId *dbFrozenXidP, MultiXactId *dbMinMultiP,
                        Oid *dbTablespace, char **dbCollate, char **dbCtype, 
char **dbIculocale,
+                       char **dbIcurules,
                        char *dbLocProvider,
                        char **dbCollversion)
 {
@@ -2715,6 +2750,14 @@ get_db_info(const char *name, LOCKMODE lockmode,
                                        else
                                                *dbIculocale = 
TextDatumGetCString(datum);
                                }
+                               if (dbIcurules)
+                               {
+                                       datum = SysCacheGetAttr(DATABASEOID, 
tuple, Anum_pg_database_daticurules, &isnull);
+                                       if (isnull)
+                                               *dbIcurules = NULL;
+                                       else
+                                               *dbIcurules = 
TextDatumGetCString(datum);
+                               }
                                if (dbCollversion)
                                {
                                        datum = SysCacheGetAttr(DATABASEOID, 
tuple, Anum_pg_database_datcollversion, &isnull);
diff --git a/src/backend/utils/adt/pg_locale.c 
b/src/backend/utils/adt/pg_locale.c
index 059e4fd79f..da514b9396 100644
--- a/src/backend/utils/adt/pg_locale.c
+++ b/src/backend/utils/adt/pg_locale.c
@@ -69,6 +69,7 @@
 
 #ifdef USE_ICU
 #include <unicode/ucnv.h>
+#include <unicode/ustring.h>
 #endif
 
 #ifdef __GLIBC__
@@ -1402,6 +1403,7 @@ struct pg_locale_struct default_locale;
 
 void
 make_icu_collator(const char *iculocstr,
+                                 const char *icurules,
                                  struct pg_locale_struct *resultp)
 {
 #ifdef USE_ICU
@@ -1418,6 +1420,35 @@ make_icu_collator(const char *iculocstr,
        if (U_ICU_VERSION_MAJOR_NUM < 54)
                icu_set_collation_attributes(collator, iculocstr);
 
+       /*
+        * If rules are specified, we extract the rules of the standard 
collation,
+        * add our own rules, and make a new collator with the combined rules.
+        */
+       if (icurules)
+       {
+               const UChar *default_rules;
+               UChar      *agg_rules;
+               UChar      *my_rules;
+               int32_t         length;
+
+               default_rules = ucol_getRules(collator, &length);
+               icu_to_uchar(&my_rules, icurules, strlen(icurules));
+
+               agg_rules = palloc_array(UChar, u_strlen(default_rules) + 
u_strlen(my_rules) + 1);
+               u_strcpy(agg_rules, default_rules);
+               u_strcat(agg_rules, my_rules);
+
+               ucol_close(collator);
+
+               status = U_ZERO_ERROR;
+               collator = ucol_openRules(agg_rules, u_strlen(agg_rules),
+                                                                 UCOL_DEFAULT, 
UCOL_DEFAULT_STRENGTH, NULL, &status);
+               if (U_FAILURE(status))
+                       ereport(ERROR,
+                                       (errmsg("could not open collator for 
locale \"%s\" with rules \"%s\": %s",
+                                                       iculocstr, icurules, 
u_errorName(status))));
+       }
+
        /* We will leak this string if the caller errors later :-( */
        resultp->info.icu.locale = MemoryContextStrdup(TopMemoryContext, 
iculocstr);
        resultp->info.icu.ucol = collator;
@@ -1580,11 +1611,19 @@ pg_newlocale_from_collation(Oid collid)
                else if (collform->collprovider == COLLPROVIDER_ICU)
                {
                        const char *iculocstr;
+                       const char *icurules;
 
                        datum = SysCacheGetAttr(COLLOID, tp, 
Anum_pg_collation_colliculocale, &isnull);
                        Assert(!isnull);
                        iculocstr = TextDatumGetCString(datum);
-                       make_icu_collator(iculocstr, &result);
+
+                       datum = SysCacheGetAttr(COLLOID, tp, 
Anum_pg_collation_collicurules, &isnull);
+                       if (!isnull)
+                               icurules = TextDatumGetCString(datum);
+                       else
+                               icurules = NULL;
+
+                       make_icu_collator(iculocstr, icurules, &result);
                }
 
                datum = SysCacheGetAttr(COLLOID, tp, 
Anum_pg_collation_collversion,
diff --git a/src/backend/utils/init/postinit.c 
b/src/backend/utils/init/postinit.c
index 2f07ca7a0e..b0e20cc635 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -421,10 +421,19 @@ CheckMyDatabase(const char *name, bool am_superuser, bool 
override_allow_connect
 
        if (dbform->datlocprovider == COLLPROVIDER_ICU)
        {
+               char       *icurules;
+
                datum = SysCacheGetAttr(DATABASEOID, tup, 
Anum_pg_database_daticulocale, &isnull);
                Assert(!isnull);
                iculocale = TextDatumGetCString(datum);
-               make_icu_collator(iculocale, &default_locale);
+
+               datum = SysCacheGetAttr(DATABASEOID, tup, 
Anum_pg_database_daticurules, &isnull);
+               if (!isnull)
+                       icurules = TextDatumGetCString(datum);
+               else
+                       icurules = NULL;
+
+               make_icu_collator(iculocale, icurules, &default_locale);
        }
        else
                iculocale = NULL;
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index c8a0bb7b3a..b2455b9c47 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -928,38 +928,52 @@ listAllDbs(const char *pattern, bool verbose)
        initPQExpBuffer(&buf);
 
        printfPQExpBuffer(&buf,
-                                         "SELECT d.datname as \"%s\",\n"
-                                         "       
pg_catalog.pg_get_userbyid(d.datdba) as \"%s\",\n"
-                                         "       
pg_catalog.pg_encoding_to_char(d.encoding) as \"%s\",\n"
-                                         "       d.datcollate as \"%s\",\n"
-                                         "       d.datctype as \"%s\",\n",
+                                         "SELECT\n"
+                                         "  d.datname as \"%s\",\n"
+                                         "  
pg_catalog.pg_get_userbyid(d.datdba) as \"%s\",\n"
+                                         "  
pg_catalog.pg_encoding_to_char(d.encoding) as \"%s\",\n",
                                          gettext_noop("Name"),
                                          gettext_noop("Owner"),
-                                         gettext_noop("Encoding"),
-                                         gettext_noop("Collate"),
-                                         gettext_noop("Ctype"));
+                                         gettext_noop("Encoding"));
        if (pset.sversion >= 150000)
                appendPQExpBuffer(&buf,
-                                                 "       d.daticulocale as 
\"%s\",\n"
-                                                 "       CASE d.datlocprovider 
WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
-                                                 gettext_noop("ICU Locale"),
+                                                 "  CASE d.datlocprovider WHEN 
'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
                                                  gettext_noop("Locale 
Provider"));
        else
                appendPQExpBuffer(&buf,
-                                                 "       NULL as \"%s\",\n"
-                                                 "       'libc' AS \"%s\",\n",
-                                                 gettext_noop("ICU Locale"),
+                                                 "  'libc' AS \"%s\",\n",
                                                  gettext_noop("Locale 
Provider"));
-       appendPQExpBufferStr(&buf, "       ");
+       appendPQExpBuffer(&buf,
+                                         "  d.datcollate as \"%s\",\n"
+                                         "  d.datctype as \"%s\",\n",
+                                         gettext_noop("Collate"),
+                                         gettext_noop("Ctype"));
+       if (pset.sversion >= 150000)
+               appendPQExpBuffer(&buf,
+                                                 "  d.daticulocale as 
\"%s\",\n",
+                                                 gettext_noop("ICU Locale"));
+       else
+               appendPQExpBuffer(&buf,
+                                                 "  NULL as \"%s\",\n",
+                                                 gettext_noop("ICU Locale"));
+       if (pset.sversion >= 160000)
+               appendPQExpBuffer(&buf,
+                                                 "  d.daticurules as 
\"%s\",\n",
+                                                 gettext_noop("ICU Rules"));
+       else
+               appendPQExpBuffer(&buf,
+                                                 "  NULL as \"%s\",\n",
+                                                 gettext_noop("ICU Rules"));
+       appendPQExpBufferStr(&buf, "  ");
        printACLColumn(&buf, "d.datacl");
        if (verbose)
                appendPQExpBuffer(&buf,
-                                                 ",\n       CASE WHEN 
pg_catalog.has_database_privilege(d.datname, 'CONNECT')\n"
-                                                 "            THEN 
pg_catalog.pg_size_pretty(pg_catalog.pg_database_size(d.datname))\n"
-                                                 "            ELSE 'No 
Access'\n"
-                                                 "       END as \"%s\""
-                                                 ",\n       t.spcname as 
\"%s\""
-                                                 ",\n       
pg_catalog.shobj_description(d.oid, 'pg_database') as \"%s\"",
+                                                 ",\n  CASE WHEN 
pg_catalog.has_database_privilege(d.datname, 'CONNECT')\n"
+                                                 "       THEN 
pg_catalog.pg_size_pretty(pg_catalog.pg_database_size(d.datname))\n"
+                                                 "       ELSE 'No Access'\n"
+                                                 "  END as \"%s\""
+                                                 ",\n  t.spcname as \"%s\""
+                                                 ",\n  
pg_catalog.shobj_description(d.oid, 'pg_database') as \"%s\"",
                                                  gettext_noop("Size"),
                                                  gettext_noop("Tablespace"),
                                                  gettext_noop("Description"));
@@ -4854,52 +4868,64 @@ listCollations(const char *pattern, bool verbose, bool 
showSystem)
        PQExpBufferData buf;
        PGresult   *res;
        printQueryOpt myopt = pset.popt;
-       static const bool translate_columns[] = {false, false, false, false, 
false, false, true, false};
+       static const bool translate_columns[] = {false, false, false, false, 
false, false, false, true, false};
 
        initPQExpBuffer(&buf);
 
        printfPQExpBuffer(&buf,
-                                         "SELECT n.nspname AS \"%s\",\n"
-                                         "       c.collname AS \"%s\",\n"
-                                         "       c.collcollate AS \"%s\",\n"
-                                         "       c.collctype AS \"%s\"",
+                                         "SELECT\n"
+                                         "  n.nspname AS \"%s\",\n"
+                                         "  c.collname AS \"%s\",\n",
                                          gettext_noop("Schema"),
-                                         gettext_noop("Name"),
+                                         gettext_noop("Name"));
+
+       if (pset.sversion >= 100000)
+               appendPQExpBuffer(&buf,
+                                                 "  CASE c.collprovider WHEN 
'd' THEN 'default' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
+                                                 gettext_noop("Provider"));
+       else
+               appendPQExpBuffer(&buf,
+                                                 "  'libc' AS \"%s\",\n",
+                                                 gettext_noop("Provider"));
+
+       appendPQExpBuffer(&buf,
+                                         "  c.collcollate AS \"%s\",\n"
+                                         "  c.collctype AS \"%s\",\n",
                                          gettext_noop("Collate"),
                                          gettext_noop("Ctype"));
 
        if (pset.sversion >= 150000)
                appendPQExpBuffer(&buf,
-                                                 ",\n       c.colliculocale AS 
\"%s\"",
+                                                 "  c.colliculocale AS 
\"%s\",\n",
                                                  gettext_noop("ICU Locale"));
        else
                appendPQExpBuffer(&buf,
-                                                 ",\n       c.collcollate AS 
\"%s\"",
+                                                 "  c.collcollate AS 
\"%s\",\n",
                                                  gettext_noop("ICU Locale"));
 
-       if (pset.sversion >= 100000)
+       if (pset.sversion >= 160000)
                appendPQExpBuffer(&buf,
-                                                 ",\n       CASE 
c.collprovider WHEN 'd' THEN 'default' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' 
END AS \"%s\"",
-                                                 gettext_noop("Provider"));
+                                                 "  c.collicurules AS 
\"%s\",\n",
+                                                 gettext_noop("ICU Rules"));
        else
                appendPQExpBuffer(&buf,
-                                                 ",\n       'libc' AS \"%s\"",
-                                                 gettext_noop("Provider"));
+                                                 "  NULL AS \"%s\",\n",
+                                                 gettext_noop("ICU Rules"));
 
        if (pset.sversion >= 120000)
                appendPQExpBuffer(&buf,
-                                                 ",\n       CASE WHEN 
c.collisdeterministic THEN '%s' ELSE '%s' END AS \"%s\"",
+                                                 "  CASE WHEN 
c.collisdeterministic THEN '%s' ELSE '%s' END AS \"%s\"",
                                                  gettext_noop("yes"), 
gettext_noop("no"),
                                                  
gettext_noop("Deterministic?"));
        else
                appendPQExpBuffer(&buf,
-                                                 ",\n       '%s' AS \"%s\"",
+                                                 "  '%s' AS \"%s\"",
                                                  gettext_noop("yes"),
                                                  
gettext_noop("Deterministic?"));
 
        if (verbose)
                appendPQExpBuffer(&buf,
-                                                 ",\n       
pg_catalog.obj_description(c.oid, 'pg_collation') AS \"%s\"",
+                                                 ",\n  
pg_catalog.obj_description(c.oid, 'pg_collation') AS \"%s\"",
                                                  gettext_noop("Description"));
 
        appendPQExpBufferStr(&buf,
diff --git a/src/include/catalog/pg_collation.h 
b/src/include/catalog/pg_collation.h
index f9d5e88faf..bfa3568451 100644
--- a/src/include/catalog/pg_collation.h
+++ b/src/include/catalog/pg_collation.h
@@ -43,6 +43,7 @@ CATALOG(pg_collation,3456,CollationRelationId)
        text            collcollate BKI_DEFAULT(_null_);        /* LC_COLLATE 
setting */
        text            collctype BKI_DEFAULT(_null_);  /* LC_CTYPE setting */
        text            colliculocale BKI_DEFAULT(_null_);      /* ICU locale 
ID */
+       text            collicurules BKI_DEFAULT(_null_);       /* ICU 
collation rules */
        text            collversion BKI_DEFAULT(_null_);        /* 
provider-dependent
                                                                                
                         * version of collation
                                                                                
                         * data */
@@ -91,6 +92,7 @@ extern Oid    CollationCreate(const char *collname, Oid 
collnamespace,
                                                        int32 collencoding,
                                                        const char 
*collcollate, const char *collctype,
                                                        const char 
*colliculocale,
+                                                       const char 
*collicurules,
                                                        const char *collversion,
                                                        bool if_not_exists,
                                                        bool quiet);
diff --git a/src/include/catalog/pg_database.h 
b/src/include/catalog/pg_database.h
index 3da3492e92..a5c4efe086 100644
--- a/src/include/catalog/pg_database.h
+++ b/src/include/catalog/pg_database.h
@@ -71,6 +71,9 @@ CATALOG(pg_database,1262,DatabaseRelationId) 
BKI_SHARED_RELATION BKI_ROWTYPE_OID
        /* ICU locale ID */
        text            daticulocale;
 
+       /* ICU collation rules */
+       text            daticurules BKI_DEFAULT(_null_);
+
        /* provider-dependent version of collation data */
        text            datcollversion BKI_DEFAULT(_null_);
 
diff --git a/src/include/utils/pg_locale.h b/src/include/utils/pg_locale.h
index cede43440b..bcadefbf08 100644
--- a/src/include/utils/pg_locale.h
+++ b/src/include/utils/pg_locale.h
@@ -95,6 +95,7 @@ typedef struct pg_locale_struct *pg_locale_t;
 extern PGDLLIMPORT struct pg_locale_struct default_locale;
 
 extern void make_icu_collator(const char *iculocstr,
+                                                         const char *icurules,
                                                          struct 
pg_locale_struct *resultp);
 
 extern pg_locale_t pg_newlocale_from_collation(Oid collid);
diff --git a/src/test/regress/expected/collate.icu.utf8.out 
b/src/test/regress/expected/collate.icu.utf8.out
index 4354dc07b8..2ac707b362 100644
--- a/src/test/regress/expected/collate.icu.utf8.out
+++ b/src/test/regress/expected/collate.icu.utf8.out
@@ -1190,6 +1190,36 @@ SELECT 'Goldmann' < 'Götz' COLLATE "de-x-icu", 
'Goldmann' > 'Götz' COLLATE tes
  t        | t
 (1 row)
 
+-- rules
+CREATE COLLATION testcoll_rules1 (provider = icu, locale = '', rules = '&a < 
g');
+CREATE TABLE test7 (a text);
+-- example from 
https://unicode-org.github.io/icu/userguide/collation/customization/#syntax
+INSERT INTO test7 VALUES ('Abernathy'), ('apple'), ('bird'), ('Boston'), 
('Graham'), ('green');
+SELECT * FROM test7 ORDER BY a COLLATE "en-x-icu";
+     a     
+-----------
+ Abernathy
+ apple
+ bird
+ Boston
+ Graham
+ green
+(6 rows)
+
+SELECT * FROM test7 ORDER BY a COLLATE testcoll_rules1;
+     a     
+-----------
+ Abernathy
+ apple
+ green
+ bird
+ Boston
+ Graham
+(6 rows)
+
+DROP TABLE test7;
+CREATE COLLATION testcoll_rulesx (provider = icu, locale = '', rules = 
'!!wrong!!');
+ERROR:  could not open collator for locale "" with rules "!!wrong!!": 
U_INVALID_FORMAT_ERROR
 -- nondeterministic collations
 CREATE COLLATION ctest_det (provider = icu, locale = '', deterministic = true);
 CREATE COLLATION ctest_nondet (provider = icu, locale = '', deterministic = 
false);
diff --git a/src/test/regress/expected/psql.out 
b/src/test/regress/expected/psql.out
index 8fc62cebd2..ba66b8a2c5 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -6170,9 +6170,9 @@ List of schemas
 (0 rows)
 
 \dO "no.such.collation"
-                            List of collations
- Schema | Name | Collate | Ctype | ICU Locale | Provider | Deterministic? 
---------+------+---------+-------+------------+----------+----------------
+                                  List of collations
+ Schema | Name | Provider | Collate | Ctype | ICU Locale | ICU Rules | 
Deterministic? 
+--------+------+----------+---------+-------+------------+-----------+----------------
 (0 rows)
 
 \dp "no.such.access.privilege"
@@ -6359,9 +6359,9 @@ cross-database references are not implemented: 
"no.such.schema"."no.such.languag
 (0 rows)
 
 \dO "no.such.schema"."no.such.collation"
-                            List of collations
- Schema | Name | Collate | Ctype | ICU Locale | Provider | Deterministic? 
---------+------+---------+-------+------------+----------+----------------
+                                  List of collations
+ Schema | Name | Provider | Collate | Ctype | ICU Locale | ICU Rules | 
Deterministic? 
+--------+------+----------+---------+-------+------------+-----------+----------------
 (0 rows)
 
 \dp "no.such.schema"."no.such.access.privilege"
@@ -6502,9 +6502,9 @@ List of text search templates
 (0 rows)
 
 \dO regression."no.such.schema"."no.such.collation"
-                            List of collations
- Schema | Name | Collate | Ctype | ICU Locale | Provider | Deterministic? 
---------+------+---------+-------+------------+----------+----------------
+                                  List of collations
+ Schema | Name | Provider | Collate | Ctype | ICU Locale | ICU Rules | 
Deterministic? 
+--------+------+----------+---------+-------+------------+-----------+----------------
 (0 rows)
 
 \dp regression."no.such.schema"."no.such.access.privilege"
diff --git a/src/test/regress/sql/collate.icu.utf8.sql 
b/src/test/regress/sql/collate.icu.utf8.sql
index b0ddc7db44..aa95c1ec42 100644
--- a/src/test/regress/sql/collate.icu.utf8.sql
+++ b/src/test/regress/sql/collate.icu.utf8.sql
@@ -472,6 +472,19 @@ CREATE COLLATION testcoll_de_phonebook (provider = icu, 
locale = 'de@collation=p
 SELECT 'Goldmann' < 'Götz' COLLATE "de-x-icu", 'Goldmann' > 'Götz' COLLATE 
testcoll_de_phonebook;
 
 
+-- rules
+
+CREATE COLLATION testcoll_rules1 (provider = icu, locale = '', rules = '&a < 
g');
+CREATE TABLE test7 (a text);
+-- example from 
https://unicode-org.github.io/icu/userguide/collation/customization/#syntax
+INSERT INTO test7 VALUES ('Abernathy'), ('apple'), ('bird'), ('Boston'), 
('Graham'), ('green');
+SELECT * FROM test7 ORDER BY a COLLATE "en-x-icu";
+SELECT * FROM test7 ORDER BY a COLLATE testcoll_rules1;
+DROP TABLE test7;
+
+CREATE COLLATION testcoll_rulesx (provider = icu, locale = '', rules = 
'!!wrong!!');
+
+
 -- nondeterministic collations
 
 CREATE COLLATION ctest_det (provider = icu, locale = '', deterministic = true);

base-commit: 2cb82e2acfba069d00c6bd253d58df03d315672a
-- 
2.39.2

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