Fix removed read-only server setting lc_collate

Relevant PG commit:
b0f6c43716
b0f6c437160db640d4ea3e49398ebc3ba39d1982

src/test/regress/expected/multi_mx_create_table.out

@@ -53,7 +53,16 @@ CREATE OPERATOR citus_mx_test_schema.=== (
     HASHES, MERGES
 );
 SET search_path TO public;
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 16 AS server_version_ge_16
+\gset
+\if :server_version_ge_16
+-- In PG16, read-only server settings lc_collate and lc_ctype are removed
+-- Relevant PG commit: b0f6c437160db640d4ea3e49398ebc3ba39d1982
+SELECT quote_ident((SELECT CASE WHEN datlocprovider='i' THEN daticulocale ELSE datcollate END FROM pg_database WHERE datname = current_database())) as current_locale \gset
+\else
 SELECT quote_ident(current_setting('lc_collate')) as current_locale \gset
+\endif
 CREATE COLLATION citus_mx_test_schema.english (LOCALE=:current_locale);
 CREATE TYPE citus_mx_test_schema.new_composite_type as (key1 text, key2 text);
 CREATE TYPE order_side_mx AS ENUM ('buy', 'sell');

src/test/regress/expected/multi_schema_support.out

@@ -347,7 +347,16 @@ SELECT * FROM nation_hash ORDER BY 1,2,3,4;
 
 --test COLLATION with schema
 SET search_path TO public;
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 16 AS server_version_ge_16
+\gset
+\if :server_version_ge_16
+-- In PG16, read-only server settings lc_collate and lc_ctype are removed
+-- Relevant PG commit: b0f6c437160db640d4ea3e49398ebc3ba39d1982
+SELECT quote_ident((SELECT CASE WHEN datlocprovider='i' THEN daticulocale ELSE datcollate END FROM pg_database WHERE datname = current_database())) as current_locale \gset
+\else
 SELECT quote_ident(current_setting('lc_collate')) as current_locale \gset
+\endif
 CREATE COLLATION test_schema_support.english (LOCALE = :current_locale);
 \c - - - :master_port
 SET citus.shard_replication_factor TO 2;

src/test/regress/sql/multi_mx_create_table.sql

@@ -51,7 +51,19 @@ CREATE OPERATOR citus_mx_test_schema.=== (
 );
 
 SET search_path TO public;
+
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 16 AS server_version_ge_16
+\gset
+
+\if :server_version_ge_16
+-- In PG16, read-only server settings lc_collate and lc_ctype are removed
+-- Relevant PG commit: b0f6c437160db640d4ea3e49398ebc3ba39d1982
+SELECT quote_ident((SELECT CASE WHEN datlocprovider='i' THEN daticulocale ELSE datcollate END FROM pg_database WHERE datname = current_database())) as current_locale \gset
+\else
 SELECT quote_ident(current_setting('lc_collate')) as current_locale \gset
+\endif
+
 CREATE COLLATION citus_mx_test_schema.english (LOCALE=:current_locale);
 
 CREATE TYPE citus_mx_test_schema.new_composite_type as (key1 text, key2 text);

src/test/regress/sql/multi_schema_support.sql

@@ -293,7 +293,19 @@ SELECT * FROM nation_hash ORDER BY 1,2,3,4;
 
 --test COLLATION with schema
 SET search_path TO public;
+
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 16 AS server_version_ge_16
+\gset
+
+\if :server_version_ge_16
+-- In PG16, read-only server settings lc_collate and lc_ctype are removed
+-- Relevant PG commit: b0f6c437160db640d4ea3e49398ebc3ba39d1982
+SELECT quote_ident((SELECT CASE WHEN datlocprovider='i' THEN daticulocale ELSE datcollate END FROM pg_database WHERE datname = current_database())) as current_locale \gset
+\else
 SELECT quote_ident(current_setting('lc_collate')) as current_locale \gset
+\endif
+
 CREATE COLLATION test_schema_support.english (LOCALE = :current_locale);
 
 \c - - - :master_port