Collations with nondeterministic comparison

This adds a flag "deterministic" to collations.  If that is false,
such a collation disables various optimizations that assume that
strings are equal only if they are byte-wise equal.  That then allows
use cases such as case-insensitive or accent-insensitive comparisons
or handling of strings with different Unicode normal forms.

This functionality is only supported with the ICU provider.  At least
glibc doesn't appear to have any locales that work in a
nondeterministic way, so it's not worth supporting this for the libc
provider.

The term "deterministic comparison" in this context is from Unicode
Technical Standard #10
(https://unicode.org/reports/tr10/#Deterministic_Comparison).

This patch makes changes in three areas:

- CREATE COLLATION DDL changes and system catalog changes to support
  this new flag.

- Many executor nodes and auxiliary code are extended to track
  collations.  Previously, this code would just throw away collation
  information, because the eventually-called user-defined functions
  didn't use it since they only cared about equality, which didn't
  need collation information.

- String data type functions that do equality comparisons and hashing
  are changed to take the (non-)deterministic flag into account.  For
  comparison, this just means skipping various shortcuts and tie
  breakers that use byte-wise comparison.  For hashing, we first need
  to convert the input string to a canonical "sort key" using the ICU
  analogue of strxfrm().

Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/flat/1ccc668f-4cbc-0bef-af67-450b47cdfee7@2ndquadrant.com

src/backend/utils/cache/catcache.c

@@ -170,13 +170,18 @@ int4hashfast(Datum datum)
 static bool
 texteqfast(Datum a, Datum b)
 {
-	return DatumGetBool(DirectFunctionCall2(texteq, a, b));
+	/*
+	 * The use of DEFAULT_COLLATION_OID is fairly arbitrary here.  We just
+	 * want to take the fast "deterministic" path in texteq().
+	 */
+	return DatumGetBool(DirectFunctionCall2Coll(texteq, DEFAULT_COLLATION_OID, a, b));
 }
 
 static uint32
 texthashfast(Datum datum)
 {
-	return DatumGetInt32(DirectFunctionCall1(hashtext, datum));
+	/* analogously here as in texteqfast() */
+	return DatumGetInt32(DirectFunctionCall1Coll(hashtext, DEFAULT_COLLATION_OID, datum));
 }
 
 static bool

src/backend/utils/cache/lsyscache.c

@@ -908,6 +908,22 @@ get_collation_name(Oid colloid)
 		return NULL;
 }
 
+bool
+get_collation_isdeterministic(Oid colloid)
+{
+	HeapTuple	tp;
+	Form_pg_collation colltup;
+	bool		result;
+
+	tp = SearchSysCache1(COLLOID, ObjectIdGetDatum(colloid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for collation %u", colloid);
+	colltup = (Form_pg_collation) GETSTRUCT(tp);
+	result = colltup->collisdeterministic;
+	ReleaseSysCache(tp);
+	return result;
+}
+
 /*				---------- CONSTRAINT CACHE ----------					 */
 
 /*