Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Jan 14, 2013 at 12:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> The old code definitely had an unreasonably large charge for indexes
>> exceeding 1e8 or so tuples. This wouldn't matter that much for simple
>> single-table lookup queries, but I could easily see it putting the
>> kibosh on uses of an index on the inside of a nestloop.
> The reported behavior was that the planner would prefer to
> sequential-scan the table rather than use the index, even if
> enable_seqscan=off. I'm not sure what the query looked like, but it
> could have been something best implemented as a nested loop w/inner
> index-scan.
Remember also that "enable_seqscan=off" merely adds 1e10 to the
estimated cost of seqscans. For sufficiently large tables this is not
exactly a hard disable, just a thumb on the scales. But I don't know
what your definition of "extremely large indexes" is.
regards, tom lane