What's the difference between these T-SQL queries using OR? - sql-server

Question

I use Microsoft SQL Server 2008 (SP1, x64). I have two queries that do the same, or so I think, but they are have completely different query plans and performance.
Query 1:
SELECT c_pk
FROM table_c
WHERE c_b_id IN (SELECT b_id FROM table_b WHERE b_z = 1)
OR c_a_id IN (SELECT a_id FROM table_a WHERE a_z = 1)
Query 2:
SELECT c_pk
FROM table_c
LEFT JOIN (SELECT b_id FROM table_b WHERE b_z = 1) AS b ON c_b_id = b_id
LEFT JOIN (SELECT a_id FROM table_a WHERE a_z = 1) AS a ON c_a_id = a_id
WHERE b_id IS NOT NULL
OR a_id IS NOT NULL
Query 1 is fast as I would expect, whereas query 2 is very slow. The query plans look quite different.
I would like query 2 to be as fast as query 1. I have software that uses query 2, and I cannot change that into query 1. I can change the database.
Some questions:
why are the query plans different?
can I "teach" SQL Server somehow that query 2 is equal to query 1?
All tables have (clustered) primary keys and proper indexes on all columns:
CREATE TABLE table_a (
a_pk int NOT NULL PRIMARY KEY,
a_id int NOT NULL UNIQUE,
a_z int
)
GO
CREATE INDEX IX_table_a_z ON table_a (a_z)
GO
CREATE TABLE table_b (
b_pk int NOT NULL PRIMARY KEY,
b_id int NOT NULL UNIQUE,
b_z int
)
GO
CREATE INDEX IX_table_b_z ON table_b (b_z)
GO
CREATE TABLE table_c (
c_pk int NOT NULL PRIMARY KEY,
c_a_id int,
c_b_id int
)
GO
CREATE INDEX IX_table_c_a_id ON table_c (c_a_id)
GO
CREATE INDEX IX_table_c_b_id ON table_c (c_b_id)
GO
The tables are not modified after filling initially. I'm the only one querying them. They contains millions of records (table_a: 5M, table_b: 4M, table_c: 12M), but using only 1% gives similar results.
Edit: I tried adding FOREIGN KEYs for c_a_id and c_b_id, but that only made query 1 slower...
I hope someone can have a look at the query plans and explain the difference.

Answer 1

Join are slower, let me say by design. First query uses a sub-query (cacheable) to filter records so it'll produce less data (and less accesses to each table).
Did you read these:
http://www.sql-server-performance.com/2006/tuning-joins/
http://blogs.msdn.com/b/craigfr/archive/2006/12/04/semi-join-transformation.aspx
What I mean is that with IN the DB can do better optimizations like removing duplicates, stop at first match and similar (and these are from school memories so I'm sure it'll do much better). So I guess the question isn't why QP is different but how smart how deep optimizations can go.

Answer 2

You are comparing non equivalent queries also you are using left join in quite unusual way.
Generally if yours intention was to select all entries in table_c which has linked records either in table_a or table_b you should use exists statement:
SELECT c_pk
FROM table_c
WHERE Exists(
SELECT 1
FROM table_b
WHERE b_z = 1 and c_b_id = b_id
) OR Exists(
SELECT 1
FROM table_a
WHERE a_z = 1 and c_a_id = a_id
)

Answer 3

Since you can't change the query, at least you can improve the query's environment.
Highlight your query, right-click on it in SSMS and select "Analyze
Query in Database Engine Tuning Advisor."
Run the analysis to find out if you need any additional indexes or
statistics built.
Heed SQL Server's advice.