Subject: Re: Allow tailoring of ICU locales with custom rules

From: Peter Eisentraut <[email-masked]>
Date: Thu, 5 Jan 2023 16:15:21 +0100

Patch needed a rebase; no functionality changes.

On 14.12.22 10:26, Peter Eisentraut wrote:
> This patch exposes the ICU facility to add custom collation rules to a 
> standard collation.  This would allow users to customize any ICU 
> collation to whatever they want.  A very simple example from the 
> documentation/tests:
> 
> CREATE COLLATION en_custom
>      (provider = icu, locale = 'en', rules = '&a < g');
> 
> This places "g" after "a" before "b".  Details about the syntax can be 
> found at 
> <https://unicode-org.github.io/icu/userguide/collation/customization/>.
> 
> The code is pretty straightforward.  It mainly just records these rules 
> in the catalog and feeds them to ICU when creating the collator object.
From ae729e2d5d37a382b7aaaa13ea95897ccb35d0c1 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <[email-masked]>
Date: Wed, 14 Dec 2022 10:15:03 +0100
Subject: [PATCH v2] 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/[email-masked]
---
 doc/src/sgml/catalogs.sgml                    | 18 +++++++
 doc/src/sgml/ref/create_collation.sgml        | 22 +++++++++
 doc/src/sgml/ref/create_database.sgml         | 12 +++++
 src/backend/catalog/pg_collation.c            |  5 ++
 src/backend/commands/collationcmds.c          | 23 +++++++--
 src/backend/commands/dbcommands.c             | 49 +++++++++++++++++--
 src/backend/utils/adt/pg_locale.c             | 41 +++++++++++++++-
 src/backend/utils/init/postinit.c             | 11 ++++-
 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/sql/collate.icu.utf8.sql     | 13 +++++
 13 files changed, 220 insertions(+), 10 deletions(-)

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 9316b811ac..afa9f28ef9 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 58f5f0cd63..2c7266107e 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 ea38c64731..aa6f121a81 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/doc/src/sgml/ref/create_database.sgml
@@ -192,6 +192,18 @@ <title>Parameters</title>
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <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>
       <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 6a4311cc63..04e259b0ba 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 518ffca09a..36ce7d8f83 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 bool have_createdb_privilege(void);
@@ -676,6 +677,7 @@ 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;
@@ -699,6 +701,7 @@ 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;
@@ -711,6 +714,7 @@ 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;
@@ -776,6 +780,12 @@ 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)
@@ -959,7 +969,7 @@ 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),
@@ -1007,6 +1017,8 @@ 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))
@@ -1098,6 +1110,9 @@ 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)
@@ -1106,6 +1121,19 @@ 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.")));
 		}
 	}
 
@@ -1314,6 +1342,10 @@ 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
@@ -1527,7 +1559,7 @@ 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)
 		{
@@ -1727,7 +1759,7 @@ 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)));
@@ -1837,7 +1869,7 @@ 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)));
@@ -2600,6 +2632,7 @@ 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)
 {
@@ -2716,6 +2749,14 @@ 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 ae5a85ed65..163119ee48 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/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 d4c8c6de38..ce9ed3c8bf 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/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: b82557ecc2ebbf649142740a1c5ce8d19089f620
-- 
2.39.0