Bug 21288: Slowness in acquisition caused by GetInvoices

Bug 21288: Slowness in acquisition caused by GetInvoices

The SQL query done by GetInvoices is quite slow on our production server,
taking about a minute.

In the query, there's this join:

  LEFT JOIN subscription ON biblio.biblionumber = subscription.biblionumber

but there's no index on subscription.biblionumber. Adding the index speeds
up the query so it only takes up about 3 seconds.

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>