Does the limit on the maximum number of SQL join tables apply to the whole query, or are subqueries counted separately?

Question

I have read about the 61 table join limit in MySQL 5, but I'm not sure how it would apply to the folllowing:

SELECT * FROM (

    SELECT * FROM tableA JOIN // Lots of other joins here...

    UNION

    SELECT * FROM tableB JOIN // Lots of other joins here...

    UNION

    SELECT * FROM tableC JOIN // Lots of other joins here...

    // etc...
)

Would I hit the limit with 61 tables in total across all the subqueries, or would it be 61 per UNIONed subquery?

Does this vary across DBs e.g. PostgreSQL, MSSQL, Oracle?

Answer 1

It seems to be 61 per subquery. Here's a fiddle demonstrating this.

http://sqlfiddle.com/#!2/2b219/5

I have a simple table with one row:

id    | value
1     | testvalue

The first query is just the table.

The second query joins the table with itself 61 times. It works fine.

The third query has a subquery containing 61 joins, which is itself joined with the table one more time. It works fine.

The fourth query joins the table 62 times. It fails.