3

I'm trying to optimize a query because the plan generates a full table scan for a subquery.

Data

The (simplified) table structure:

CREATE TABLE RECORD (
  ID INTEGER PRIMARY KEY AUTO_INCREMENT
);

CREATE TABLE POINT (
  ID INTEGER PRIMARY KEY AUTO_INCREMENT,
  RECORD_ID INTEGER NOT NULL,
  CONSTRAINT POINT_FK1 FOREIGN KEY (RECORD_ID) REFERENCES RECORD(ID)
);

CREATE TABLE SEGMENT (
  ID INTEGER PRIMARY KEY AUTO_INCREMENT,
  POINT_START INTEGER NOT NULL,
  POINT_END INTEGER NOT NULL,
  RECORD_ID INTEGER NOT NULL,
  CONSTRAINT SEGMENT_FK1 FOREIGN KEY (POINT_START) REFERENCES POINT(ID),
  CONSTRAINT SEGMENT_FK2 FOREIGN KEY (POINT_END) REFERENCES POINT(ID),
  CONSTRAINT SEGMENT_FK3 FOREIGN KEY (RECORD_ID) REFERENCES RECORD(ID)
);

The data (in real life the tables have hundreds of thousands of rows):

INSERT INTO RECORD(ID) VALUES (1), (2), (3);
INSERT INTO POINT(ID, RECORD_ID) VALUES (1, 1), (2, 1), (3, 1), (4, 1), (5, 2), (6, 2);
INSERT INTO SEGMENT(ID, POINT_START, POINT_END, RECORD_ID) VALUES (1, 1, 2, 1), (2, 3, 4, 1);

Query

The (simplified) original query:

SELECT
  RECORD.ID AS RECORD_ID,
  POINT.ID AS POINT_ID
FROM
  RECORD
  LEFT OUTER JOIN POINT
    ON RECORD.ID = POINT.RECORD_ID
    AND (NOT EXISTS (SELECT 1 FROM SEGMENT WHERE POINT_START = POINT.ID OR POINT_END = POINT.ID))

Results:

+-----------+----------+
| RECORD_ID | POINT_ID |
+-----------+----------+
|         1 |     NULL |
|         2 |        5 |
|         2 |        6 |
|         3 |     NULL |
+-----------+----------+

This has a query plan with a full table scan for the subquery:

+------+--------------------+---------+-------+-------------------------+-----------+---------+----------------+------+--------------------------+
| id   | select_type        | table   | type  | possible_keys           | key       | key_len | ref            | rows | Extra                    |
+------+--------------------+---------+-------+-------------------------+-----------+---------+----------------+------+--------------------------+
|    1 | PRIMARY            | RECORD  | index | NULL                    | PRIMARY   | 4       | NULL           |    3 | Using index              |
|    1 | PRIMARY            | POINT   | ref   | POINT_FK1               | POINT_FK1 | 4       | TEST.RECORD.ID |    1 | Using where; Using index |
|    2 | DEPENDENT SUBQUERY | SEGMENT | ALL   | SEGMENT_FK1,SEGMENT_FK2 | NULL      | NULL    | NULL           |    2 | Using where              |
+------+--------------------+---------+-------+-------------------------+-----------+---------+----------------+------+--------------------------+

The intent of the query is to return at least 1 row per RECORD. If there are POINTs that don't belong to a SEGMENT, return each of those on their own row. The results are processed like this:

cur_record = nil
for row in results
    if (cur_record == nil) or (row['RECORD_ID'] != cur_record.id)
        cur_record = createRecord()
        cur_record.id = row['RECORD_ID']
    end
    if row['POINT_ID'] != nil
        point = createPoint()
        point.id = row['POINT_ID']
        cur_record.addPoint(point)
    end
end

Current improved query

The best query I've come up with so far is:

SELECT
  RECORD.ID AS RECORD_ID,
  POINT.ID AS POINT_ID
FROM
  RECORD
  LEFT OUTER JOIN POINT
    ON RECORD.ID = POINT.RECORD_ID
    AND POINT.ID NOT IN (SELECT POINT_START FROM SEGMENT WHERE POINT_START = POINT.ID)
    AND POINT.ID NOT IN (SELECT POINT_END FROM SEGMENT WHERE POINT_END = POINT.ID)

This has the query plan:

+------+--------------------+---------+-------+---------------+-------------+---------+----------------+------+--------------------------+
| id   | select_type        | table   | type  | possible_keys | key         | key_len | ref            | rows | Extra                    |
+------+--------------------+---------+-------+---------------+-------------+---------+----------------+------+--------------------------+
|    1 | PRIMARY            | RECORD  | index | NULL          | PRIMARY     | 4       | NULL           |    3 | Using index              |
|    1 | PRIMARY            | POINT   | ref   | POINT_FK1     | POINT_FK1   | 4       | TEST.RECORD.ID |    1 | Using where; Using index |
|    3 | DEPENDENT SUBQUERY | SEGMENT | ref   | SEGMENT_FK2   | SEGMENT_FK2 | 4       | TEST.POINT.ID  |    1 | Using where; Using index |
|    2 | DEPENDENT SUBQUERY | SEGMENT | ref   | SEGMENT_FK1   | SEGMENT_FK1 | 4       | TEST.POINT.ID  |    1 | Using where; Using index |
+------+--------------------+---------+-------+---------------+-------------+---------+----------------+------+--------------------------+

This causes it to use the FK indexes, which makes the query a lot faster. However, it has two dependent subqueries.

Is there a better query I could use?

1
  • 1
    Seems like 2 subqueries with simple join conditions perform better than 1 subquery with a complex condition (complex meaning; OR). Not unexpected really.Test with larger table sizes and keep the best of them. May 3, 2016 at 16:22

2 Answers 2

2

I tried the LEFT JOIN .. IS NULL idea I noted in the comment:

SELECT
  RECORD.ID AS RECORD_ID,
  POINT.ID AS POINT_ID
FROM
  RECORD
  LEFT OUTER JOIN (
    POINT
    LEFT JOIN SEGMENT s1 ON POINT.ID = s1.POINT_START
    LEFT JOIN SEGMENT s2 ON POINT.ID = s2.POINT_END
  ) ON RECORD.ID = POINT.RECORD_ID
      AND s1.POINT_START IS NULL AND s2.POINT_END IS NULL;

http://sqlfiddle.com/#!9/f6e9ca/1

The dependent subqueries are now joins instead, using the same indexes but it should have less overhead - dependent subquery means firing a subquery per main (outer) select row which is not as well optimized as joins are.

1

If indexes exist on SEGMENT (POINT_START), on SEGMENT (POINT_END), and on POINT (RECORD_ID), your query would perform well. MySQL 5+ creates indexes automatically for any foreign keys, so they're most likely indexed.

Note that IN is used for sets, but your query would return zero or one records at most for in IN clause, so an EXISTS would perform better:

SELECT
  RECORD.ID AS RECORD_ID,
  POINT.ID AS POINT_ID
FROM
  RECORD
  LEFT OUTER JOIN POINT
    ON RECORD.ID = POINT.RECORD_ID
    AND NOT EXISTS (SELECT 1 FROM SEGMENT WHERE POINT_START = POINT.ID)
    AND NOT EXISTS (SELECT 1 FROM SEGMENT WHERE POINT_END = POINT.ID)
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.