Confirm very large decimals round-trip

No code changes are needed, but very large denominators in a Rat are fragile. This test ensures DBIish doesn't accidentally overflow the Rat.

t/34-pg-types.t

@@ -2,7 +2,7 @@ use v6;
 use Test;
 use DBIish::CommonTesting;
 
-plan 2;
+plan 3;
 
 my %con-parms;
 # If env var set, no parameter needed.
@@ -48,3 +48,16 @@ subtest 'Test changing column type' => {
     ($col1) = $sth.row;
     is $col1, 'changed', 'Changed';
 }
+
+subtest 'Big Decimals' => {
+    my FatRat @values = '0.00000000000000000000123'.FatRat,
+                        '1.123456789123456789123456789123456789123456789123456789'.FatRat;
+    for @values -> $val {
+        my $sth = $dbh.execute('SELECT ?::numeric', $val);
+        my ($col1) = $sth.row;
+        isa-ok $col1, Rat, 'Rational';
+
+        # Convert to FatRat to prevent comparison issues during stringification that is (via eq) uses.
+        is $col1.FatRat, $val.FatRat, 'Expected value roundtripped';
+    }
+}