slow performance due to OR condition on separate tables

[Jose-Miguel Pulido]

I have the following tables: products, product_categories, categories, product_tags and tags

Given a keyword that can either be a category name or a tag name (not known in advance), I want to find all products that either have a category with that name, or a tag with that name. I am using the query:

SELECT
p.product_id
FROM
products p, product_categories pc, categories c, product_tags pt, tags t
WHERE
(
p.product_id = pc.product_id
AND
pc.category_id = c.category_id
AND
MATCH(c.name) against ('keyword_here')
)
OR
(
p.product_id = pt.product_id
AND
pt.tag_id = t.tag_id
AND
MATCH(t.name) AGAINST ('keyword_here')
)


the query is slow. explain shows that it ends up visiting all rows in tables categories and tags. Indices are set correctly.

If I perform join decomposition and I break the query into two queries, one to search categories and another to search tags, performance improves significantly.

Anyone sees a way to achieve good performance in a single query?

Thanks