How can I speed up this search?

I am searching on a FULL JOIN of two tables (using 7.2.1). VACUUM
ANALYZE has been done.

If I search on either of them separately, the search uses the
appropriate index, but when they are joined, the second table uses a
sequential scan and takes far longer than (I think) it ought. Can I
reshape the query or add additional indexes to improve the planner's
guess?

bray=# explain analyze SELECT '' AS van,
bray-# COALESCE(a.year, f.year) AS year,
bray-# COALESCE(a.month, f.month) AS month,
bray-# COALESCE(f.qty,0) AS fc_qty, COALESCE(a.qty,0) AS
qty, COALESCE(a.home_inv_qty,0) AS home_inv_qty,
bray-# COALESCE(a.exp_inv_qty,0) AS exp_inv_qty,
bray-# COALESCE(f.revenue,0) AS revenue,
bray-# COALESCE(a.ccy, 'GBP') AS ccy,
bray-# COALESCE(a.val, 0.) AS val,
bray-# COALESCE(a.home_inv_val, 0.) AS home_inv_val,
bray-# COALESCE(a.exp_inv_val, 0.) AS exp_inv_val
bray-# FROM stock_allocation AS a
bray-# FULL JOIN sales_forecast AS f
bray-# ON a.product = f.product AND
bray-# a.year = f.year AND
bray-# a.month = f.month AND
bray-# a.ccy = f.ccy
bray-# WHERE a.product = 'CC009' OR f.product = 'CC009';
NOTICE: QUERY PLAN:

Merge Join (cost=49104.90..51498.34 rows=241423 width=101) (actual
time=31518.32..33565.86 rows=48 loops=1)
-> Index Scan using sales_forecast_pkey on sales_forecast f
(cost=0.00..1543.00 rows=27528 width=40) (actual time=0.27..278.38
rows=27528 loops=1)
-> Sort (cost=49104.90..49104.90 rows=241423 width=61) (actual
time=29576.37..30185.22 rows=241423 loops=1)
-> Seq Scan on stock_allocation a (cost=0.00..5580.23
rows=241423 width=61) (actual time=0.07..2772.26 rows=241423
loops=1)
Total runtime: 38289.59 msec

EXPLAIN
bray=# \d stock_allocation
Table "stock_allocation"
Column | Type | Modifiers
--------------+-----------------------+--------------------
product | character varying(10) | not null
year | smallint | not null
month | smallint | not null
ccy | character(3) | not null
qty | integer | not null default 0
val | numeric(12,2) | default 0
home_inv_qty | integer | default 0
home_inv_val | numeric(12,2) | default 0
exp_inv_qty | integer | default 0
exp_inv_val | numeric(12,2) | default 0
Indexes: product_currency_index
Primary key: stock_allocation_pkey
Check constraints: "month in range" (("month" >= 1) AND
(((float8("year") < date_part('year'::text,
('now'::text)::timestamp(6) with time zone)) AND ("month" <= 12)) OR
((float8("year") = date_part('year'::text,
('now'::text)::timestamp(6) with time zone)) AND (float8("month") <=
date_part('month'::text, ('now'::text)::timestamp(6) with time
zone)))))
"year in range" (("year" > 1987) AND
(float8("year") <= date_part('year'::text,
('now'::text)::timestamp(6) with time zone)))
Triggers: RI_ConstraintTrigger_26246616,
RI_ConstraintTrigger_26246610

bray=# \d stock_allocation_pkey
Index "stock_allocation_pkey"
Column | Type
---------+-----------------------
product | character varying(10)
year | smallint
month | smallint
ccy | character(3)
unique btree (primary key)

bray=# \d product_currency_index
Index "product_currency_index"
Column | Type
---------+-----------------------
product | character varying(10)
ccy | character(3)
btree

bray=# explain analyze select * from stock_allocation where product
= 'CC009';
NOTICE: QUERY PLAN:

Index Scan using product_currency_index on stock_allocation
(cost=0.00..204.90 rows=68 width=61) (actual time=0.43..1.33 rows=48
loops=1)
Total runtime: 1.54 msec

EXPLAIN
bray=# \d sales_forecast
Table "sales_forecast"
Column | Type | Modifiers
---------+-----------------------+-----------
product | character varying(10) | not null
year | integer | not null
month | integer | not null
ccy | character(3) | not null
qty | integer | not null
revenue | numeric(12,2) | not null
Primary key: sales_forecast_pkey
Check constraints: "must be sterling" (ccy = 'GBP'::bpchar)
"month in range" (("month" >= 1) AND ("month" <=
12))
"year in range" (("year" > 2001) AND
(float8("year") < (date_part('year'::text, date('now'::text)) + 3)))
Triggers: RI_ConstraintTrigger_26246754

bray=# \d sales_forecast_pkey
Index "sales_forecast_pkey"
Column | Type
---------+-----------------------
product | character varying(10)
year | integer
month | integer
ccy | character(3)
unique btree (primary key)


bray=# explain select * from sales_forecast where product = 'CC009';
NOTICE: QUERY PLAN:

Index Scan using sales_forecast_pkey on sales_forecast
(cost=0.00..92.57 rows=25 width=40)

EXPLAIN

Forcing use of the index in the join does not help; it takes slightly
longer, so the planner is correct. With ENABLE_SEQSCAN off:

NOTICE: QUERY PLAN:

Merge Join (cost=544454.29..546847.73 rows=241423 width=101)
(actual time=31523.99..33582.48 rows=48 loops=1)
-> Index Scan using sales_forecast_pkey on sales_forecast f
(cost=0.00..1543.00 rows=27528 width=40) (actual time=0.26..283.75
rows=27528 loops=1)
-> Sort (cost=544454.29..544454.29 rows=241423 width=61) (actual
time=29593.48..30177.42 rows=241423 loops=1)
-> Index Scan using product_currency_index on
stock_allocation a (cost=0.00..500929.62 rows=241423 width=61)
(actual time=35.60..3539.31 rows=241423 loops=1)
Total runtime: 38485.57 msec

EXPLAIN

It seems to me that something is being done in the wrong order, since
there are actually 25 rows to be joined with 48, but the planner is
expecting to join 27528 rows with 241423. Should it not plan to do the
selection before doing the join? Or what am I missing?

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839 932A 614D 4C34 3E1D 0C1C

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