Remember last collation to speed up collation cache.

This optimization is to avoid a performance regression in an upcoming
patch that will remove lc_collate_is_c().

Discussion: https://postgr.es/m/96a559be83329bc66074a3925ebcfa8ceb16dfc5.camel@j-davis.com
Discussion: https://postgr.es/m/646f662e145ab38cff1c04d475f4448f53fc5042.camel@j-davis.com
Discussion: https://postgr.es/m/54565933-d82f-4d7c-8f47-288b1b570fd8@eisentraut.org

src/backend/utils/adt/pg_locale.c

@@ -151,6 +151,13 @@ typedef struct
 static MemoryContext CollationCacheContext = NULL;
 static collation_cache_hash *CollationCache = NULL;
 
+/*
+ * The collation cache is often accessed repeatedly for the same collation, so
+ * remember the last one used.
+ */
+static Oid	last_collation_cache_oid = InvalidOid;
+static pg_locale_t last_collation_cache_locale = NULL;
+
 #if defined(WIN32) && defined(LC_MESSAGES)
 static char *IsoLocaleName(const char *);
 #endif
@@ -1570,6 +1577,9 @@ pg_newlocale_from_collation(Oid collid)
 	if (collid == DEFAULT_COLLATION_OID)
 		return &default_locale;
 
+	if (last_collation_cache_oid == collid)
+		return last_collation_cache_locale;
+
 	cache_entry = lookup_collation_cache(collid);
 
 	if (cache_entry->locale == 0)
@@ -1695,6 +1705,9 @@ pg_newlocale_from_collation(Oid collid)
 		cache_entry->locale = resultp;
 	}
 
+	last_collation_cache_oid = collid;
+	last_collation_cache_locale = cache_entry->locale;
+
 	return cache_entry->locale;
 }