Question

I hold a set of nodes in one mysql table1 and a table of edges in another one (table2). Nodes come with primary keys and edges use this “foreign key”

**table1**
id label
1  node1
2  node2
3  node3

**table2**
FK_first FK_sec   rel
1        3        guardian
2        1        guardian
1        3        times

I know the db-design is not perfect, but its simple…
Now i want the number of ‘rel’ for every node and do a query like:

SELECT 
  label, 
  COUNT( rel ) as freq
FROM
  `table1` 
  LEFT JOIN table2 ON (id=FK_first OR id=FK_second) 
GROUP BY label
ORDER BY freq DESC

I have about 1000 nodes and 2000 edges. A query with ON (id=FK_first OR id=FK_second), then the query is way faster (<1 sec). The other query needs about 6 sec which is ver slow.
I would appreciate some comments to speed this up a bit 🙂

1. LEFT JOIN table2 ON (id=FK_first OR id=FK_second) ~6 sec
2. LEFT JOIN table2 ON (id=FK_first) ~0.16 sec

3. LEFT JOIN table2 ON (id=FK_second) ~0.16 sec

4. LEFT JOIN table2 ON id IN (FK_first,FK_second) ~6 sec

EXPLAIN 1:
        id  select_type table   type    possible_keys                   key key_len ref rows    Extra
        1   SIMPLE          table1  ALL NULL                            NULL    NULL    NULL    2571    Using temporary; Using filesort
        1   SIMPLE          table2  ALL FK_first,FK_second,FK_first_2   NULL    NULL    NULL    3858    

EXPLAIN 2:
id  select_type  table  type   possible_keys       key    key_len ref        rows   Extra
1   SIMPLE       table1 index  NULL                PRIMARY    2   NULL       2571   Using index; Using temporary; Using filesort
1   SIMPLE       table2 ref   FK_first,FK_first_2  FK_first_2 4   table1.id  1

Answer 1

Try doing two joins and moving the “OR” into the COUNT() function:

For every row, this joins table2 once on FK1, then again on FK2 (if it is not already joined to that row via FK1. Then in the COUNT, we specify that only rows which have either join’s rel column as non-null.

SELECT 
  label, 
  COUNT( table2A.rel || table2B.rel ) as freq
FROM
  `table1` 
  LEFT JOIN 
    table2 as table2A 
    ON id=table2A.FK_first
  LEFT JOIN 
    table2 as table2B 
    ON id=table2B.FK_second 
    AND  table2A.FKFirst != table2B.FKFirst
GROUP BY label
ORDER BY freq DESC