Issue in Improving the performance using prepared plan

Hi,

I have written following trigger and trying to improve the performance by
using prepared query everytime. I have used spi_prepare to prepare the query
and $_SHARED global hash to persist the prepared plan but it doesn't seem to
work. Though $query will be same always in following trigger, it prepares
query everytime and never uses prepared plan.
Could anyone tell me what's wrong going on?

CREATE OR REPLACE FUNCTION techdb_table_trigger()
RETURNS trigger AS
$BODY$
our ($id, $query, $plan, $change_log_table);

$change_log_table = "ChangeLogTable";

$id = $_TD->{new}{'id'};

$query = (<<ENDQUERY);
INSERT INTO $change_log_table(id)
SELECT \$1
EXCEPT SELECT id
FROM $change_log_table
WHERE id = \$1
AND txid = txid_current()
AND txtime = transaction_timestamp();
ENDQUERY

if (exists($_SHARED{$query})) {
$plan = $_SHARED{$query};
* elog(INFO, "########## Preparing the query ###########"); --> Always
comes here. Don't know why?*
} else {
$plan = spi_prepare($query, 'INTEGER');
$_SHARED{$query} = $plan;
* elog(INFO, "###############Using already prepared the
query##############"); --> Never comes here.*
}

spi_exec_prepared($plan, $id);

$BODY$
LANGUAGE 'plperl' VOLATILE SECURITY DEFINER

Thanks,
Jignesh