Optimize for N+1 problem [WIP] about walkable

[myguidingstar]

• start a tree whenever it's an ident key
• look at children of all level in ast, siege for join keys
• build and run the join queries
• map & filter instead of join-seq
• join keys' reader with the key being ::p/path

[myguidingstar]

the problem with solving SQL N+1 problem using SELECT ... WHERE id IN(1, 11, 111) is that we can't limit number of rows per id which can become a disaster if the table is big. Even if we try to use LIMIT in that query, chances are some ids will take up the limit while others are left out.

For the case where some limit is specified, the SQL query will be UNION of the generated queries:

(SELECT ... WHERE id = 1 LIMIT 5)
UNION
(SELECT ... WHERE id = 11 LIMIT 5)
UNION
(SELECT ... WHERE id = 111 LIMIT 5)

[myguidingstar]

walkable's internals will need refactor a bit to allow merging multiple sql queries with params
See #27

[myguidingstar]

for compatibility reason, the following UNION query will be used

SELECT * FROM (SELECT ... WHERE id = 1 LIMIT 5)
UNION
SELECT * FROM (SELECT ... WHERE id = 11 LIMIT 5)
UNION
SELECT * FROM (SELECT ... WHERE id = 111 LIMIT 5)

[myguidingstar]

should keep track and limit the number of nested levels, default to 3. When the limit is reached, start a new tree again