Significantly improved index query performance [skip ci]

test/t/007_stages.pl

@@ -0,0 +1,38 @@
+use strict;
+use warnings;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 2;
+
+# Initialize node
+my $node = get_new_node('node');
+$node->init;
+$node->start;
+
+# Create table
+$node->safe_psql("postgres", "CREATE EXTENSION vector;");
+$node->safe_psql("postgres", "CREATE TABLE tst (i int4, v vector(3));");
+$node->safe_psql("postgres",
+	"INSERT INTO tst SELECT i, ARRAY[i % 1000, i % 1000, i % 1000] FROM generate_series(1, 10000) i;"
+);
+
+my @limits = (128, 2048);
+my @expected = ();
+
+foreach (@limits) {
+	my $res = $node->safe_psql("postgres", "SELECT i, v FROM tst ORDER BY v <-> '[0,0,0]', i LIMIT $_;");
+	push(@expected, $res);
+}
+
+$node->safe_psql("postgres", "CREATE INDEX ON tst USING ivfflat (v) WITH (lists = 5);");
+
+for my $i (0 .. $#limits) {
+	my $res = $node->safe_psql("postgres", qq(
+		SET enable_seqscan = off;
+		SET ivfflat.probes = 5;
+		WITH tmp AS (
+			SELECT *, v <-> '[0,0,0]' AS d FROM tst ORDER BY v <-> '[0,0,0]' LIMIT $limits[$i]
+		) SELECT i, v FROM tmp ORDER BY d, i;
+	));
+	is($res, $expected[$i]);
+}