如何优化SQL自连接查询？

Question

I've got two tables (and these are part of a third-party application, so I can't change their schemas):

• doc has doc_id, is_active, part_number, name, and description
• node has node_id, doc_id, and parent_node_id.

node.doc_id refers to a doc.doc_id value (there isn't a foreign key relationship) and node.parent_node_id refers to a node.node_id value, setting up a parent/child relationship of the values in the doc table. Each entry in the doc table can have zero or one parent, and any number of children.

For a given part number, I need the name and description for all matching entries in the doc table AND (here's the tricky part) for each such matching entry I need to know whether that entry has any active children.

Here's an example:

doc
doc_id   is_active   part_number   name   description
1        T           AAA           Fred   Little
2        T           AAA           George Middle
3        T           AAA           Sam    Morse
4        T           CCC           Mary   Moo
5        T           DDD           Carol  Smith
6        F           DDD           Midge  Moo

node
node_id    doc_id    parent_node_id
10          1           null
11          2           null
12          3           null
13          4           10
14          5           10
15          6           11

So you can see that doc_id 1 has 2 children (doc_ids 4 and 5) and doc_id 2 has one child (doc_id 6).

Graphically:

doc[doc_id=1] -> node[doc_id=1,node_id=10]; nodes with parent_node_id=10 are node[node_id=13,doc_id=4] and node[node_id=14,doc_id=5]; both doc[doc_id=4] and doc[doc_id=5] have is_active=T.

doc[doc_id=2] -> node[doc_id=2,node_id=11]; the only node with parent_node_id=11 is node[node_id=15,doc_id=6] but doc[doc_id=6] has is_active=F.

If I make my request for part_number=AAA, I need to get back:

doc_id   name    description    has_active_children
1        Fred    Little         T
2        George  Middle         F
3        Sam     Morse          F

Right now I've got this query where I count the number of children (which is unnecessary but the only thing I could figure out):

select d1.*,
   (select count(dn.node_id) from node dn
    inner join doc dc on dn.doc_id=dc.doc_id
    where dn.parent_node_id=
      (select dx.node_id from node dx where dx.doc_id=d1.doc_id)
    and dc.is_active='T') as childCount
from doc d1 where d1.part_number='AAA'

This works but isn't terribly fast. We're running on SQL Server, and I tried the "set showplan_all" but didn't understand the output well enough to make any changes.

Is there an obviously better way to do this query? Or, is there a document that would help me understand the showplan output?

Answer 1

This should be a decent starting point. I've had to do similar self-joins in a database that held a hierarchical representation of geographical regions.

If you change either of the 2 left join statements into a simple join , any parent that doesn't have a child will be removed from the query results.

SELECT parent.[doc_id],
       parent.[name],
       parent.[description],
       parent.[part_number],
       CASE WHEN COUNT(child.[doc_id]) > 0 THEN 'T' ELSE 'F' END
FROM       doc   parent
JOIN       node  parentRef on parent.[doc_id] = parentRef.[doc_id]
LEFT JOIN  node  childRef on parentRef.[node_id] = childRef.[parent_node_id]
LEFT JOIN  doc   child on child.[doc_id] = childRef.[doc_id]
WHERE parent.[part_number] = 'AAA'
GROUP BY parent.[doc_id], parent.[name], parent.[part_number], parent.[description]

EDIT: Adding part_number to the query

Also, as with any SQL query, look into the Indexes that exist on these tables. You may be able to add an Index or two to increase the query performance.

Answer 2

You can use Window Function which will be a Good fit for your case.

SELECT parent.[doc_id],
       parent.[name],
       parent.[description],
       parent.[part_number],
       CASE WHEN ROW_NUMBER(PARTITION BY child.[doc_id]) > 0 THEN 'T' ELSE 'F' END  AS childCount
FROM       doc   parent
JOIN       node  parentRef on parent.[doc_id] = parentRef.[doc_id]
LEFT JOIN  node  childRef on parentRef.[node_id] = childRef.[parent_node_id]
LEFT JOIN  doc   child on child.[doc_id] = childRef.[doc_id]
WHERE parent.[part_number] = 'AAA' AND child.is_active = 'T'