I am building a database containing information on university courses.
Each course can be associated with
- one or many authors
- one or many disciplines
- one or many institutions
- one or many levels
My database contains the following tables:
- course (cou_id, cou_name, cou_number, cou_year, cou_term)
- author (aut_id, aut_last)
- discipline (dis_id, dis_name)
- institution (ins_id, ins_name, ins_classification)
- level (lev_id, lev_name)
- authorcourse (linking table)
- coursediscipline (linking table)
- courseinstitution (linking table)
- courselevel (linking table)
In order to retrieve ALL courses from the database (and the corresponding author,discipline, institution, and level information), I use the following query:
SELECT DISTINCT aut_last, c.cou_id, cou_name, cou_number, cou_year, cou_term, dis_name, ins_name, ins_classification, lev_name
FROM authorcourse ac1
INNER JOIN authorcourse ac2
ON ac1.cou_id = ac2.cou_id
INNER JOIN author a
ON ac2.aut_id=a.aut_id
INNER JOIN course c
ON ac2.cou_id = c.cou_id
INNER JOIN coursediscipline cd1
ON ac2.cou_id = cd1.cou_id
INNER JOIN coursediscipline cd2
ON cd1.cou_id = cd2.cou_id
INNER JOIN discipline d
ON cd2.dis_id = d.dis_id
INNER JOIN courseinstitution ci1
ON ac2.cou_id = ci1.cou_id
INNER JOIN courseinstitution ci2
ON ci1.cou_id = ci2.cou_id
INNER JOIN institution i
ON ci2.ins_id = i.ins_id
INNER JOIN courselevel cl1
ON ac2.cou_id = cl1.cou_id
INNER JOIN courselevel cl2
ON cl1.cou_id = cl2.cou_id
INNER JOIN level l
ON cl2.lev_id = l.lev_id
This query works well when there are 15 courses in the database with “simple” relationships. e.g.:
cou_name = 'course1', cou_number = 'C1', cou_year = '1999', cou_term = 'summer'
aut_last = 'Doe1'
dis_name = 'discipline1'
ins_name = 'institution1', ins_classification = 'classification1'
lev_name = 'level1'
–>Showing rows 0 – 14 ( 15 total, Query took 0.0118 sec)
EXPLAIN produces the following table:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE ac1 index cou_id aut_id 2 NULL 15 Using index; Using temporary
1 SIMPLE ac2 ref PRIMARY,aut_id,cou_id cou_id 2 ccdb.ac1.cou_id 1 Using index
1 SIMPLE a eq_ref PRIMARY PRIMARY 2 ccdb.ac2.aut_id 1
1 SIMPLE c eq_ref PRIMARY PRIMARY 2 ccdb.ac2.cou_id 1 Using where
1 SIMPLE cd1 ref PRIMARY,cou_id PRIMARY 2 ccdb.ac1.cou_id 1 Using index
1 SIMPLE cd2 ref PRIMARY,cou_id,dis_id PRIMARY 2 ccdb.ac2.cou_id 1 Using where; Using index
1 SIMPLE d eq_ref PRIMARY PRIMARY 2 ccdb.cd2.dis_id 1
1 SIMPLE ci1 ref PRIMARY,cou_id PRIMARY 2 ccdb.ac2.cou_id 1 Using where; Using index
1 SIMPLE ci2 ref PRIMARY,cou_id,ins_id PRIMARY 2 ccdb.ac2.cou_id 1 Using where; Using index
1 SIMPLE i eq_ref PRIMARY PRIMARY 2 ccdb.ci2.ins_id 1
1 SIMPLE cl1 ref PRIMARY,cou_id PRIMARY 2 ccdb.cd1.cou_id 1 Using where; Using index
1 SIMPLE cl2 ref PRIMARY,cou_id,lev_id PRIMARY 2 ccdb.cl1.cou_id 1 Using where; Using index
1 SIMPLE l eq_ref PRIMARY PRIMARY 2 ccdb.cl2.lev_id 1
Problem: Performance dramatically decreases when there are 15 courses with multiple relationships. Example course:
cou_name = 'course1', cou_number = 'C1', cou_year = '1999', cou_term = 'summer'
aut_last = 'Doe1', 'Doe', 'Doe3', 'Doe4'
dis_name = 'discipline1', 'discipline2', 'discipline3', 'discipline4'
ins_name = 'institution1'(ins_classification = 'classification1'), 'institution2'(ins_classification = 'classification2'), 'institution3'(ins_classification = 'classification3'), 'institution4' (ins_classification = 'classification4')
lev_name = 'level1', 'level2', 'level3', 'level4'
–>Showing rows 0 – 29 ( 3,840 total, Query took 14.7039 sec)
EXPLAIN produces the following table:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE c ALL PRIMARY NULL NULL NULL 15 Using temporary
1 SIMPLE ac1 ref PRIMARY,aut_id,cou_id cou_id 2 ccdb.c.cou_id 2 Using index
1 SIMPLE a eq_ref PRIMARY PRIMARY 2 ccdb.ac1.aut_id 1
1 SIMPLE ac2 ref cou_id cou_id 2 ccdb.c.cou_id 2 Using index
1 SIMPLE cd1 ref PRIMARY,cou_id cou_id 2 ccdb.ac1.cou_id 2 Using where; Using index
1 SIMPLE cd2 ref PRIMARY,cou_id,dis_id cou_id 2 ccdb.c.cou_id 2 Using index
1 SIMPLE d eq_ref PRIMARY PRIMARY 2 ccdb.cd2.dis_id 1
1 SIMPLE ci1 ref PRIMARY,cou_id cou_id 2 ccdb.ac1.cou_id 2 Using where; Using index
1 SIMPLE ci2 ref PRIMARY,cou_id,ins_id cou_id 2 ccdb.ac2.cou_id 2 Using where; Using index
1 SIMPLE i eq_ref PRIMARY PRIMARY 2 ccdb.ci2.ins_id 1
1 SIMPLE cl1 ref PRIMARY,cou_id cou_id 2 ccdb.c.cou_id 2 Using index
1 SIMPLE cl2 ref PRIMARY,cou_id,lev_id cou_id 2 ccdb.ci2.cou_id 2 Using where; Using index
1 SIMPLE l eq_ref PRIMARY PRIMARY 2 ccdb.cl2.lev_id 1
When run through my PHP website, I get the following error “Fatal error: Maximum execution time of 30 seconds exceeded in …”
Question: How can I speed up this query?
I tried several different combinations of the joins and
(as you can see in the EXPLAIN results) I indexed all columns I considered potentially relevant.
Any help would be greatly appreciated.
IT looks to me asif your pulling all this data for the ‘course view’ type detail page ?
If so I would say, once a course has been made in the database, how often are the number of authors, disciplines, institutions and levels going to change ?
IF there never going to change from the time there set, then when it is set, also set it in a totally denormalised table like this:
courseView (cou_id, cou_name, cou_number, cou_year, cou_term, data)
.. and in ‘data’, you just put in a serialised array of all the data. Ugly, but it’s going to be fast.
Then, when you search by course id to pull up the one, you can search only one row, one index and pull up all the data instantly.
..
Also, if your going to let people search by Authors, then you can still do this with the normalised table with a simple query like normal.