Hi Mike,
Thanks for you help!
Best,
Jia
mos wrote:
Jia,
The code you sent seems to be able to get the job done.
You could try something simpler by executing 2 sql statements instead
of using one. Something like:
create table rmpdata1
select ri.*, mv.* from RItime as ri left join MVtime as mv on
(ri.code=mv.code and ri.ndate=mv.ndate));
insert into rmpdata1
select ri.*, mv.* from MVtime as mv left join RItime as ri on
(ri.code=mv.code and ri.ndate=mv.ndate)) where ri.code is null and
ri.date is null;
You will of course have to play with the column list to avoid
duplicate columns. The Insert statement will insert rows from mv that
are missing in ri.
Mike
At 01:51 PM 9/6/2009, Jia Chen wrote:
Hi Mike,
Thanks for your detailed answer. Now, I understand what you mean.
And, yes, I agree with you that keeping all data in one table works
better for a bunch of 1:1 relationship tables. Actually, this is
what I was trying to do with that query.
Since you mention "They all had a 1:1 relationship and occasionally
some of the tables did not have a corresponding row." and "From then
on I've merged all 8 tables into one and if any of the subordinate
table data isn't available for a row, its columns are set to NULL", I
do want to ask you about how you set the columns to NULL for rows in
subordinate table data unavailable because I have similar situation.
If I want to combine two tables into one, I think that a full outer
join can achieve what you did. However, MySQL does not have full
outer join. So, I am using
create table rmpdata1
(select ri.*, mv.MV, coalesce(ri.code,mv.code) as ccode,
coalesce(ri.ndate,mv.ndate) as cndate
from
RItime as ri left outer join MVtime as mv
on
(ri.code=mv.code and ri.ndate=mv.ndate))
union
(select ri.*, mv.MV, coalesce(ri.code,mv.code) as ccode,
coalesce(ri.ndate,mv.ndate) as cndate
from
RItime as ri right outer join MVtime as mv
on
(ri.code=mv.code and ri.ndate=mv.ndate));
This query takes more than twice as much time as the query in my
first e-mail. Do you have a better way? Thanks.
Best,
Jia
mos wrote:
Jia,
Yes, it is a 1:1 relationship between table RItime and MVtime.
However, I don't get your suggestion, "I'd recommend joining the
two tables into 1 table so you don't have to join them in the first
place." Could you elaborate that?
Sure but first I have to relate it to my own experience. I had 8
tables of around 25 million rows each. They all had a 1:1
relationship and occasionally some of the tables did not have a
corresponding row. I felt it was better from a design point of view
to have 8 different tables and do the joins on the tables that I
needed for each of my particular queries. I'd be joining anywhere
from 2 to 5 or 6 or even all 8 tables at a time, using a where
clause to select 15k rows at a time. This is the way to do it from a
normalized point of view. All of the information is in its
respective table and only assemble the tables for each particular
query.
Well, this was sloooowww! A heck of a lot of work was done to join
the tables together on a 2 column key (like yours). I also had to
run maintenance on the tables to see which tables where corrupted or
were missing rows that should have been there. The tables also
repeated columns from the other tables like date and product_id that
is used to help identify each row. Well to make a long story short,
it was far too much effort to juggle the relationships between all
of these tables.
Then a colleague made the monumental announcement by saying "I've
never found the need to use more than 1 table when there was a 1:1
relationship. There is a tremendous speed cost involved in piecing
the data back together. I put all of the data into 1 table". So the
light went on for me. From then on I've merged all 8 tables into one
and if any of the subordinate table data isn't available for a row,
its columns are set to NULL, which is the values they would have had
anyway after a left join.
I am perfectly happy know with one wide table with over 100 columns.
Everything is in its place and maintenance is a dream. Queries are
also quite fast because all of the information is under one table
and not 8. I don't have to worry about optimizing the indexes for
the table joins because there aren't any joins between these tables
because it is all in 1 row.
So you really have to ask yourself, why spend 10 minutes each time
your query is run? Instead you eliminate the query altogether by
keeping the data of the 2 tables into 1 table in the first place.
Mike
At 09:45 AM 9/6/2009, Jia Chen wrote:
Thanks for your reply, Mike.
Yes, 13419851 rows were added to rmpdata1. However, 10 minutes seem
to be too long. I run the same join by using SQL procedure in a
statistical software called SAS on a similar machine. It only takes
1 minute and 3 seconds.
Yes, it is a 1:1 relationship between table RItime and MVtime.
However, I don't get your suggestion, "I'd recommend joining the
two tables into 1 table so you don't have to join them in the first
place." Could you elaborate that?
I was using B-tree index. Switching to HASH does help. Now, it
takes 4 min 50.17 sec to run the query.
I also turn on profiling by using
mysql> set profiling = 1;
Query OK, 0 rows affected (0.01 sec)
After the query finishes, I get
mysql> show profile;
+----------------------+------------+
| Status | Duration |
+----------------------+------------+
| starting | 0.000123 |
| checking permissions | 0.000010 |
| Opening tables | 0.000044 |
| System lock | 0.000007 |
| Table lock | 0.000011 |
| init | 0.000083 |
| creating table | 0.003428 |
| After create | 0.000124 |
| System lock | 0.000004 |
| Table lock | 0.000051 |
| optimizing | 0.000007 |
| statistics | 0.000033 |
| preparing | 0.000020 |
| executing | 0.000004 |
| Sending data | 290.153530 |
| end | 0.000008 |
| end | 0.000004 |
| query end | 0.000003 |
| freeing items | 0.000010 |
| closing tables | 0.000025 |
| logging slow query | 0.000001 |
| logging slow query | 0.013429 |
| cleaning up | 0.000004 |
+----------------------+------------+
23 rows in set (0.02 sec)
MySQL spends most of its time sending data. According to
http://dev.mysql.com/doc/refman/5.0/en/general-thread-states.html,
sending data means that "the thread is processing rows for a
|SELECT| <http://dev.mysql.com/doc/refman/5.0/en/select.html>
statement and also is sending data to the client." Is there more
room to optimize this query? Thanks again.
Best,
Jia
mos wrote:
How many rows were added to rmpdata1 table? If it is 13.4 million
rows then it is going to take several minutes to join this many
rows from the 2 tables.
Is there a 1:1 relationship between the two tables or a 1:Many? If
there is a 1:1 then I'd recommend joining the two tables into 1
table so you don't have to join them in the first place.
The only other thing I can suggest is to change the type of index
on the tables being joined to see if that makes a speed
difference. For example, if you are using BTREE then switch to
HASH or vice versa. See
http://dev.mysql.com/doc/refman/5.0/en/create-index.html for more
info.
Mike
At 10:05 AM 9/5/2009, Jia Chen wrote:
Hi there,
One simple query took more than 10 minutes. Here is how relevant
rows in the slow query log looks like:
# Time: 090905 10:49:57
# u...@host: root[root] @ localhost []
# Query_time: 649 Lock_time: 0 Rows_sent: 0 Rows_examined:
26758561
use world;
create table rmpdata1 select ri.*,
mv.MV, coalesce(ri.code,mv.code) as ccode,
coalesce(ri.ndate,mv.ndate) as cndate
from RItime as ri left outer join
MVtime as mv
on (ri.code=mv.code and
ri.ndate=mv.ndate);
When I "explain" only the select clause, I get
------------+----------+-------+
| id | select_type | table | type | possible_keys | key |
key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------------------------+----------+-------+
| 1 | SIMPLE | ri | ALL | NULL | NULL | NULL
| NULL | 13419851 | |
| 1 | SIMPLE | mv | eq_ref | PRIMARY | PRIMARY | 11
| world.ri.code,world.ri.ndate | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------------------------+----------+-------+
2 rows in set (0.00 sec)
I use "show table status from world;" to get information about
two tables, RItime and MVtime, in the join clause:
Name: RItime
Engine: MyISAM
Version: 10
Row_format: Dynamic
Rows: 13419851
Avg_row_length: 31
Data_length: 427721848
Max_data_length: 281474976710655
Index_length: 347497472
Data_free: 0
Auto_increment: NULL
Create_time: 2009-09-03 10:17:57
Update_time: 2009-09-03 12:04:02
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
*************************** 2. row ***************************
Name: MVtime
Engine: MyISAM
Version: 10
Row_format: Dynamic
Rows: 13562373
Avg_row_length: 31
Data_length: 430220056
Max_data_length: 281474976710655
Index_length: 350996480
Data_free: 0
Auto_increment: NULL
Create_time: 2009-09-03 13:31:33
Update_time: 2009-09-03 13:43:51
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
I also describe these two tables:
mysql> desc RItime;
+-------+------------+------+-----+------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+------------+-------+
| code | varchar(6) | NO | PRI | | |
| ndate | date | NO | PRI | 0000-00-00 | |
| ri | double | YES | | NULL | |
| time | date | YES | | NULL | |
| bdate | date | YES | | NULL | |
+-------+------------+------+-----+------------+-------+
5 rows in set (0.00 sec)
mysql> desc MVtime;
+-------+------------+------+-----+------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+------------+-------+
| code | varchar(6) | NO | PRI | | |
| ndate | date | NO | PRI | 0000-00-00 | |
| MV | double | YES | | NULL | |
| time | date | YES | | NULL | |
| bdate | date | YES | | NULL | |
+-------+------------+------+-----+------------+-------+
5 rows in set (0.00 sec)
Could you give me some hint on how to improve the speed of this
query?
Thanks.
Best,
Jia
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