pg: Minor docs + tests

FU.xs

@@ -205,6 +205,7 @@ void cache(fupg_st *x, ...)
     FU::Pg::st::text_results = FUPG_TEXT_RESULTS
     FU::Pg::st::text         = FUPG_TEXT
   CODE:
+    if (ix == 0 && x->prepared) fu_confess("Invalid attempt to change statement configuration after it has already been prepared or executed");
     FUPG_STFLAGS;
 
 void param_types(fupg_st *st)

FU/Pg.pm

@@ -105,6 +105,15 @@ Connection is dead or otherwise unusable.
 
 Set the default settings for new statements created with B<< $conn->q() >>.
 
+=item B<< $conn->cache_size($num) >>
+
+Set the number of prepared statements to keep in the cache. Defaults to 256.
+
+Setting this (temporarily) to 0 will immediately reclaim all cached statements.
+Prepared statements that still have an active C<$st> object are not counted
+towards this number. The cache works as an LRU: when it's full, the statement
+that hasn't been used for the longest time is reclaimed.
+
 =item B<< $conn->disconnect >>
 
 Close the connection. Any active transactions are rolled back and any further
@@ -455,7 +464,7 @@ as well.  Much older versions will certainly not work fine.
 =item * Only supports the UTF-8 encoding for all text strings sent to and
 received from the PostgreSQL server. The encoding is assumed to be UTF-8 by
 default, but if this may not be the case in your situation, setting
-`client_encoding=utf8` as part of the connection string or manually switching
+C<client_encoding=utf8> as part of the connection string or manually switching
 to it after C<connect()> is always safe:
 
   my $conn = FU::Pg->connect('');

t/pgconnect.t

@@ -47,8 +47,15 @@ subtest '$st prepare & exec', sub {
         my $st = $conn->q('SELECT 1');
         is_deeply $st->param_types, [];
         is_deeply $st->columns, [{ name => '?column?', oid => 23 }];
+
+        ok !eval { $st->cache; 1 };
+        like $@, qr/Invalid attempt to change statement configuration/;
+        $st = $st->text;
+
         is $conn->exec('SELECT 1 FROM pg_prepared_statements'), 1;
+
         is $st->exec, 1;
+
         ok !eval { $st->exec; 1 };
         like $@, qr/Invalid attempt to execute statement multiple times/;
     }
@@ -79,6 +86,19 @@ subtest '$st prepare & exec', sub {
     okerr FATAL => exec => qr/unexpected status code/;
 
     is $conn->q('SET client_encoding=utf8')->exec, undef;
+
+    ok !eval { $conn->q('select 1; select 2')->exec; 1 };
+    okerr ERROR => prepare => qr/cannot insert multiple commands into a prepared statement/;
+
+    # Interleaved
+    {
+        my $a = $conn->q('SELECT 1 as a');
+        my $b = $conn->q('SELECT 2 as b');
+        is_deeply $a->columns, [ { oid => 23, name => 'a' } ];
+        is_deeply $b->columns, [ { oid => 23, name => 'b' } ];
+        is $a->val, 1;
+        is $b->val, 2;
+    }
 };
 
 subtest '$st->val', sub {