usage of indexes for inner joins

I fear this has been asked many times about PostgreSQL, and I have read
the docs about how indexes are supposed to be defined and used, but I
don't understand why the engine and optimizer is doing what it does in
the simplest of situations. Is it that its tuning is heavily data
dependent?

My case of interest is more complicated, but I decided to create a toy
case to try to understand. Here it is:

-- Table "foo" DDL

CREATE TABLE "public"."foo"(

"projectid" int4 NOT NULL ,

"uid" int4 NOT NULL ,

"name" varchar(254) NOT NULL ,

"ver" varchar(127) NOT NULL ,

"startdate" date NOT NULL ,

"enddate" date NOT NULL ,

"status" varchar(254) NOT NULL ,

"percentdone" numeric(7,2) NOT NULL ,

"championuid" int4 NOT NULL ,

"pmuid" int4 NOT NULL ,

PRIMARY KEY ("projectid")

) WITHOUT OIDS;

-- Table "bignum" DDL

CREATE TABLE "public"."bignum"(

"thing" numeric(100) NOT NULL

) WITHOUT OIDS;

CREATE INDEX "t" ON "public"."bignum" USING btree ("thing");

Running

EXPLAIN ANALYZE SELECT A.* FROM bignum B, foo A WHERE A.projectid
= B.thing;

yields:

Nested Loop (cost=0.00..15.51 rows=1 width=407) (actual
time=0.041..0.041 rows=0 loops=1)

Join Filter: ((a.projectid)::numeric = b.thing) ->

Seq Scan on bignum b (cost=0.00..1.01 rows=1 width=16) (actual
time=0.024..0.027 rows=1 loops=1) ->

Seq Scan on foo a (cost=0.00..11.80 rows=180 width=407) (actual
time=0.003..0.003 rows=0 loops=1)

Total runtime: .169 ms ;

Like *how* *come*? There are indexes on both columns of the join. Is
it the NUMERIC datatype messing things up? Unlikely, as I've seen the
same with INTEGERs.

If it is data dependent (these tables are presently empty), any
suggestions as to how to tune a database for unknown mixes of data?

This is run on the Windows version of PG, but I'm seeing the same kind
of thing on Linux.

Thanks.