Index usage

We're having a problem with one of our queries being slow. It appears to be due
to the index being used to go from tableA to tableB.

Here are the tables:
CREATE TABLE tableA
(
table_idA int8 NOT NULL DEFAULT nextval('tableA_id_seq'::regclass),
CONSTRAINT table_idA_pk PRIMARY KEY (table_idA),
)
WITHOUT OIDS;

CREATE TABLE tableB
(
table_idB int8 NOT NULL DEFAULT nextval('tableB_id_seq'::regclass),
table_idA int8 NOT NULL,
direction char NOT NULL,
CONSTRAINT tableB_pk PRIMARY KEY (table_idB),
CONSTRAINT tableB_unq UNIQUE (table_idA, direction),
)
WITHOUT OIDS;

CREATE TABLE last_summarized
(
summary_name varchar(64) NOT NULL,
summarized_id int8,
max_session_id int8,
CONSTRAINT last_summarized_pk PRIMARY KEY (summary_name)
)
WITHOUT OIDS;

Here is the query:
explain
SELECT * FROM
last_summarized ls
JOIN tableA s ON s.table_idA > ls.summarized_id AND s.table_idA
<= ls.max_session_id
LEFT JOIN tableB sf ON s.table_idA = sf.table_idA AND sf.direction = 'a'::"char"
LEFT JOIN tableB sfb ON s.table_idA = sfb.table_idA AND sfb.direction = 'b'::"char"
WHERE ls.summary_name::text = 'summary'::text

Size of tables in # of rows
tableA: 9,244,816
tableB: 15,398,497
last_summarized: 1

Explain of the above query:
"Hash Left Join (cost=1811349.31..18546527.89 rows=1029087 width=294)"
" Hash Cond: ("outer".table_idA = "inner".table_idA)"
" -> Hash Left Join (cost=915760.88..7519203.61 rows=1029087 width=219)"
" Hash Cond: ("outer".table_idA = "inner".table_idA)"
" -> Nested Loop (cost=0.00..126328.57 rows=1029087 width=144)"
" -> Index Scan using last_summarized_pk on last_summarized ls (cost=0.00..5.98 rows=1 width=82)"
" Index Cond: ((summary_name)::text = 'summary'::text)"
" -> Index Scan using table_idA_pk on tableA s (cost=0.00..110886.29 rows=1029087 width=62)"
" Index Cond: ((s.table_idA > "outer".summarized_id) AND (s.table_idA <= "outer".max_session_id))"
" -> Hash (cost=784763.16..784763.16 rows=8100289 width=75)"
" -> Bitmap Heap Scan on tableB sf (cost=216418.55..784763.16 rows=8100289 width=75)"
" Recheck Cond: (direction = 'a'::"char")"
" -> Bitmap Index Scan on tableB_unq (cost=0.00..216418.55 rows=8100289 width=0)"
" Index Cond: (direction = 'a'::"char")" <------ USING part of Index
" -> Hash (cost=775968.61..775968.61 rows=7396725 width=75)"
" -> Bitmap Heap Scan on tableB sfb (cost=216418.55..775968.61 rows=7396725 width=75)"
" Recheck Cond: (direction = 'b'::"char")"
" -> Bitmap Index Scan on tableB_unq (cost=0.00..216418.55 rows=7396725 width=0)"
" Index Cond: (direction = 'b'::"char")" <------ USING part of Index

From the above explain see inline comment("<------ USING part of Index"). The table_idA column
looks like it is being ignored in the index Cond. If I enable sequential scan the Index Cond in
question gets replaced with a Seq scan.

Also if I disable enable_bitmapscan sometimes both columns of the index(tableB_unq) will be
used.

Does anyone know why we're experiencing this behavior?