Expensive where clause

Hi All,

I have been working on a SQL statement that contains a WHERE clause of the
form WHERE column1 > column2. The query runs pretty quickly (285ms) without
the WHERE clause but slows to a relative crawl (5850ms) when it is included.
Anu suggestions on how to improve the performance would be greatly appreciated.

Kind Regards,
Keith

SELECT all_shipped_items.item_id,
sum (all_shipped_items.quantity) AS quantity
--Get the inventory items and the last date they were counted.
FROM (
SELECT items.id AS item_id,
COALESCE(last_inventory.inventory_date, CAST('0001-01-01' AS
date)) AS inventory_date
FROM peachtree.tbl_item AS items
LEFT OUTER JOIN ( SELECT DISTINCT ON ( inventory.tbl_data.item_id)
inventory.tbl_data.item_id,
inventory.tbl_detail.inventory_date
FROM inventory.tbl_data
INNER JOIN inventory.tbl_detail
ON ( inventory.tbl_data.inventory_id =
inventory.tbl_detail.inventory_id )
ORDER BY inventory.tbl_data.item_id,
inventory.tbl_data.inventory_id DESC
) AS last_inventory
ON ( items.id = last_inventory.item_id )
WHERE ( NOT items.inactive )
AND items.item_class = 1 -- stock item
AND items.item_type IN ( 'DIR', 'NET' )
) AS all_items
--Get the inventory items and the date they were shipped from the invoices.
RIGHT OUTER JOIN (
-- Get the direct items from tbl_line_item.
SELECT invoice.tbl_line_item.quantity,
invoice.tbl_line_item.item_id,
invoice.tbl_detail.ship_date
FROM invoice.tbl_line_item
JOIN peachtree.tbl_item
ON ( invoice.tbl_line_item.item_id = peachtree.tbl_item.id )
JOIN invoice.tbl_detail
ON ( invoice.tbl_line_item.i_number =
invoice.tbl_detail.i_number )
WHERE ( NOT peachtree.tbl_item.inactive )
AND peachtree.tbl_item.item_class = 1 -- stock item
AND peachtree.tbl_item.item_type = 'DIR'
UNION ALL
-- Get the assembly items from tbl_line_item.
SELECT invoice.tbl_line_item.quantity *
peachtree.tbl_assembly.quantity
AS quantity,
peachtree.tbl_assembly.component_id AS item_id,
invoice.tbl_detail.ship_date
FROM invoice.tbl_line_item
JOIN peachtree.tbl_assembly
ON ( invoice.tbl_line_item.item_id =
peachtree.tbl_assembly.id )
JOIN peachtree.tbl_item
ON ( invoice.tbl_line_item.item_id = peachtree.tbl_item.id )
JOIN invoice.tbl_detail
ON ( invoice.tbl_line_item.i_number =
invoice.tbl_detail.i_number )
WHERE ( NOT peachtree.tbl_item.inactive )
AND peachtree.tbl_item.item_type = 'ASY'
UNION ALL
-- Get the direct items from tbl_item_bom.
SELECT merged_invoice.quantity *
sales_order.tbl_item_bom.quantity
AS quantity,
sales_order.tbl_item_bom.item_id,
merged_invoice.ship_date
FROM sales_order.tbl_item_bom
JOIN ( SELECT invoice.tbl_detail.i_number,
invoice.tbl_detail.so_number,
invoice.tbl_detail.ship_date,
invoice.tbl_line_item.i_line,
invoice.tbl_line_item.quantity,
invoice.tbl_line_item.item_id
FROM invoice.tbl_detail
JOIN invoice.tbl_line_item
ON ( invoice.tbl_detail.i_number =
invoice.tbl_line_item.i_number )
) AS merged_invoice
ON ( sales_order.tbl_item_bom.number =
merged_invoice.so_number AND
sales_order.tbl_item_bom.line =
merged_invoice.i_line )
JOIN peachtree.tbl_item
ON ( sales_order.tbl_item_bom.item_id = peachtree.tbl_item.id )
WHERE ( NOT peachtree.tbl_item.inactive )
AND peachtree.tbl_item.item_class = 1 -- stock item
AND peachtree.tbl_item.item_type IN ( 'DIR', 'NET' )
UNION ALL
-- Get the assembly items from tbl_item_bom.
SELECT merged_invoice.quantity *
sales_order.tbl_item_bom.quantity *
peachtree.tbl_assembly.quantity
AS quantity,
peachtree.tbl_assembly.component_id AS item_id,
merged_invoice.ship_date
FROM sales_order.tbl_item_bom
JOIN ( SELECT invoice.tbl_detail.i_number,
invoice.tbl_detail.so_number,
invoice.tbl_detail.ship_date,
invoice.tbl_line_item.i_line,
invoice.tbl_line_item.quantity,
invoice.tbl_line_item.item_id
FROM invoice.tbl_detail
JOIN invoice.tbl_line_item
ON ( invoice.tbl_detail.i_number =
invoice.tbl_line_item.i_number )
) AS merged_invoice
ON ( sales_order.tbl_item_bom.number =
merged_invoice.so_number AND
sales_order.tbl_item_bom.line =
merged_invoice.i_line )
JOIN peachtree.tbl_assembly
ON ( sales_order.tbl_item_bom.item_id =
peachtree.tbl_assembly.id )
JOIN peachtree.tbl_item
ON ( sales_order.tbl_item_bom.item_id = peachtree.tbl_item.id )
WHERE ( NOT peachtree.tbl_item.inactive )
AND peachtree.tbl_item.item_type = 'ASY'
) AS all_shipped_items
ON ( all_items.item_id = all_shipped_items.item_id )
WHERE all_shipped_items.ship_date > all_items.inventory_date
GROUP BY all_shipped_items.item_id