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SELECT 
  product_id 
FROM 
  cscart_product_bundle_product_links 
WHERE 
  bundle_id = 1868 
  AND show_on_product_page = 'N'

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_product_bundle_product_links	ref	bundle_id	bundle_id	4	const	21	Using where