最佳连接两个MySQL表

Question

I have a table (T1) with ca. 500000 non duplicate records:

ID1    Relation  ID2
4      Rel4      13
5      Rel5       4
13     Rel13     16
16     Rel16     5

I have the properties table T1_Prop:

ID    Entity    
4     Ent4     
5     Ent5
13    Ent13   
16    Ent16  

I want to join these two tables (based on id : 4) in an efficient way as follows:

 Entity   Relation   Entity
 Ent4      Rel4      Ent13  
 Ent5      Rel5      EntEnt4

I designed this select statement including JOIN which works fine. However, I am not sure if this the best way to do:

select 
  a.entity, 
  r.relation, 
  b.entity 
from T1 as r 
INNER JOIN T1_Prop as a ON a.ID=r.ID1 AND (r.ID1=4 OR r.ID2=4) 
INNER JOIN T1_Prop as b ON b.ID=r.ID2;

Answer 1

This is a fine use of SQL. It's built for this kind of query.

You'll need two covering indexes to speed this up, on T1 . They are:

(ID1, ID2, relation)

and

(ID2, ID1, relation)

The two indexes are for handling the OR clause. It is the only potential performance issue I see, and that's just because OR operations sometimes trick the query planner into doing too much table scanning.

Try refactoring your query to this to make your selection of ID values more visible.

select   a.entity, r.relation, b.entity 
  from T1 as r 
 INNER JOIN T1_Prop as a ON a.ID=r.ID1  
 INNER JOIN T1_Prop as b ON b.ID=r.ID2
 WHERE (r.ID1=4 OR r.ID2=4) 

Then, if you have trouble with performance, after you create the covering indexes, refactor it again to

select   a.entity, r.relation, b.entity 
  from T1 as r 
 INNER JOIN T1_Prop as a ON a.ID=r.ID1  
 INNER JOIN T1_Prop as b ON b.ID=r.ID2
 WHERE r.ID1=4 
UNION
select   a.entity, r.relation, b.entity 
  from T1 as r 
 INNER JOIN T1_Prop as a ON a.ID=r.ID1  
 INNER JOIN T1_Prop as b ON b.ID=r.ID2
 WHERE r.ID2=4 

Answer 2

Your query looks fine except for the first ON clause. The condition (r.ID1=4 OR r.ID2=4) is not a rule for which record from T1_Prop to join to the T1 record. It is rather a condition, which T1 records to consider and belongs hence in the WHERE clause.

select 
  a.entity AS entity1, 
  r.relation, 
  b.entity AS entity2
FROM t1 AS r 
INNER JOIN t1_prop AS a ON a.id = r.id1
INNER JOIN t1_prop AS b ON b.id = r.id2
WHERE r.id1 = 4 OR r.id2 = 4;

This won't change the execution plan; the DBMS will execute this just the same. But it's more readable as it shows the actual intention: get relations where one of the IDs is 4 and join the entities to those relations.

Another option to show this intention is:

select 
  a.entity AS entity1, 
  r.relation, 
  b.entity AS entity2
FROM (SELECT * FROM t1 WHERE r.id1 = 4 OR r.id2 = 4) AS r 
INNER JOIN t1_prop AS a ON a.id = r.id1
INNER JOIN t1_prop AS b ON b.id = r.id2;

Some consider subqueries in FROM less readable, but, well, others don't. And when queries get more complex and say you even deal with aggregates from different tables, this is often the way to go and build a clean query.

Neither of above queries is actually better or worse than the other.