Okay so maybe this is way deeper than I will ever need to go however I want to be able to analyze this nested loop so that I can understand it.
Given:
mysql> describe t1;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| dt | datetime | NO | MUL | NULL | |
+-------+----------+------+-----+---------+-------+
1 row in set (0.00 sec)
And:
mysql> insert t1 values(101),(102),(103),(104),(105),(106),(107),(#c:=now());
Query OK, 8 rows affected (0.03 sec)
Records: 8 Duplicates: 0 Warnings: 0
And:
mysql> insert t1 select #c:=#c+interval 1 second from t1,t1 b,t1 c,t1 d,t1 e,t1 f;
Query OK, 262144 rows affected (1.94 sec)
Records: 262144 Duplicates: 0 Warnings: 0
So far I have the understanding that (#ofrows)^(#oftables)=(# of rows Added)
My question is why this is the case. I cannot find out exactly how MySQL is handling the rows and other system variables in order to create the equation that I have provided here. My equation is clearly a simplified version of the resulting action performed by the server as using 2 rows of data and 6 tables similarly gives an output of 64.
Does anyone know exactly how this is manipulated? I have been working on this for 2 days and I cannot get my mind off of it...
Also why is it inserting more than 6... maybe 36? rows into the table in the first place?? it is only specifying one possible select-able row from the tables and that is the previously inserted now() and then adding 1 second to that row and resetting the #c based on the final change so shouldn't it by logic only insert a few rows?
I guess to put it simply I understand what is happening specifically in the select #c:=#c+interval 1 second portion of the statement, but after that I am not quite sure...
I guess to put it simply how does:
select #c:=#c+interval 1 second;
+--------------------------+
| #c:=#c+interval 1 second |
+--------------------------+
| 2014-07-20 18:17:50 |
+--------------------------+
1 row in set (0.00 sec)
turn into this:
...
Query OK, 262144 rows affected (1.94 sec)
Records: 262144 Duplicates: 0 Warnings: 0
The answer that was satisfactory for me on this issue was so simple I can't believe that I didn't realize it before now.
Today I needed to quickly fill a table with values so I decided to use another table that I had to do this. For example take table i which has the column i with values 1-50. I executed the following command to populate my table c column i.
insert into c select i.i from i,i b;
Based on my knowledge above I know that this would fill table c with exactly 2500 values. being that it executed the 50^2.
What I actually found that this was doing was examining each row from table i and multiplying it by the count of rows found in the first buffered instance i b and then inserting the resulting rows into the table. If you have more buffered instances it will get the results from one and then proceed to multiply those results further by the original 50 rows that would be found in the instance (for example i c).
I ended up with 50 rows of 1-50 within table c.
I didn't realize that this match based multiplication was being performed in this way before because I was using an incrementing operator and it would incrementally create new rows based on this multiplication, however it did not leave any indication that this is what it did due to the fact that it was not matching rows to duplicate rather it matched the number of rows to insert.
Related
I have a circumstance where I need to update some table rows, marking the ones that do not appear in an external data-source as disabled (i.e. update active=0). The straight-forwards solution is to BEGIN a transaction, UPDATE every row to active=0, and then scan the remote data, doing an UPDATE for each entry that should be active=1 to put it back. I have around 1k rows, so this should be a relatively quick operation, even if there is a lot of inefficient query parsing.
However, this data will often not change at all. Hence, in the majority of cases, the net effects of the transaction will be zero change. If the database engine will resolve the whole thing, detect that nothing is changing, and not change anything as a result, that would be ideal. However, if it is going to go through and actually update every row, every time, I would rather find another solution.
Here's a demo. I created a table with just a simple integer in a row.
mysql> create table t ( i int );
mysql> insert into t set i = 42;
I check the current number of log writes.
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432152 |
+---------------------------+---------+
Then change the value in the row with an UPDATE and confirm it resulted in a log write:
mysql> update t set i = 43;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432153 |
+---------------------------+---------+
Next, make an UPDATE that has no net effect.
mysql> update t set i = 43;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0
Notice Changed: 0.
Look at the log writes, it is also unchanged:
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432153 |
+---------------------------+---------+
I think it has pretty much been concluded that there is disk I/O for your no-op. Let's discuss the task at hand:
Instead of actually modifying the database, can you keep in memory a list of the items that might be disabled? After you have made the scan, if there are any to disable, then proceed to disable all in a single UPDATE ... WHERE id IN (...)
On another topic... If you are actually doing this
BEGIN;
UPDATE a=0; -- for all rows
COMMIT;
-- all are disabled briefly
BEGIN;
UPDATE a=1 WHERE id = ... -- one row at a time
COMMIT;
Then you have a window where everthing is disabled. You probably don't want that.
I built an administrative system in Laravel for my office, using mysql, one of the main tables is for the price of the products which is a double. The system has been in production for over 2 years now, but the government here changed our currency and removed 5 zeroes, so I need to change all the price values while keeping them with some decimals.
Currently, the lowest price in the list is 500
While the highest is 14985010
I need to perform a query to change ALL the price values in that column on the production DB (backup is done, so should be fine)
So the 500 one should be: 0.005
And the 14985010 should be: 149.8501
I'm thinking that all that is needed is to divide the values by 100000, but i might be wrong.
My SQL skills are super rusty, and I've been searching for a while but couldn't find the right answer. Any pointer would be greatly appreciated. Thanks!
I actually would recommend against just dividing all your currency data by some large factor, e.g. 100K. Instead, I propose adding a new column to your prices table, which is key into another table giving the factor which should be used to determine the current price. So you might have a setup like this:
prices table
price | factorKey
500 | 2
14985010 | 2
factors table
factorKey | value
1 | 1
2 | 0.00001
Then, to generate the actual current prices, you could do a join:
SELECT
p.price * f.value AS price
FROM prices p
INNER JOIN factors f
ON p.factorKey = f.factorKey;
Under this scheme, you would only need to modify the current factor to be used with your price data. You could even maintain another table to keep track of the historical prices. One reason to suggest this is that perhaps your government will change prices again in the future. It is error prone to do such large manual divisions/multiplications on your original data.
You should first fix the column to be sure it can handle the appropriate decimal places. I would suggest something like:
alter table t modify column price decimal(38, 10);
Then just update the value:
update t
price = price / 100000;
Having said that, you might want to check if the values are exactly what you expect:
select cast(cast(price as decimal(38, 10)) / 100000 as decimal(38, 10))
There may be subtle rounding errors that are not to your liking.
First of all, your currency data type should support decimal numbers, not just integers. I would suggest DECIMAL(12,4) for your column.
After you are sure the column data type can hold decimal numbers, you can update all rows in the table.
Here's a demonstration:
mysql> create table MyTable ( MyCurrencyColumn decimal(12,4));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into MyTable values (500), (14985010);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from MyTable;
+------------------+
| MyCurrencyColumn |
+------------------+
| 500.0000 |
| 14985010.0000 |
+------------------+
2 rows in set (0.00 sec)
mysql> UPDATE MyTable SET MyCurrencyColumn = MyCurrencyColumn / 100000;
Query OK, 2 rows affected (0.02 sec)
Rows matched: 2 Changed: 2 Warnings: 0
mysql> select * from MyTable;
+------------------+
| MyCurrencyColumn |
+------------------+
| 0.0050 |
| 149.8501 |
+------------------+
I have my time data stored in a MySQL Row called "timestart" and I have three rows containing the values 9:00am, 10:00am, 1:00pm and I am trying to pull these out in a sorted order using the STR_TO_DATE function in my MySQL query. Here is the statement I am using:
SELECT * FROM db.test WHERE name='Name' and dateusing='2015-10-21' ORDER BY STR_TO_DATE('timeusingstart', '%h:%i%p');
It runs successfully but the rows get returned in the order of 10:00am, 9:00am, 1:00pm when I need them to be sorted in the time sequential order. I used the %h:%i%p based on the information I found at this documentation. Can anyone tell me what I'm doing wrong here? Thanks very much!
You're not running STR_TO_DATE() on the column, you're running it on a string. Try removing the quotes from around timeusingstart and it should work.
By the way, MySQL provides excellent documentation of all their functions!
mysql> create table test (test varchar(255));
Query OK, 0 rows affected (0.06 sec)
mysql> insert into test values('9:00am'), ('10:00am'), ('1:00pm');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from test order by str_to_date(test, '%h:%i%p');
+---------+
| test |
+---------+
| 9:00am |
| 10:00am |
| 1:00pm |
+---------+
3 rows in set (0.01 sec)
Your statement is nearly correct. You need to remove the quotes around first parameter for STR_TO_DATE, it is parsing a string you provided, not a value of the column - timeusingstart.
STR_TO_DATE(timeusingstart, '%h:%i%p');
I am doing these :
insert into table_name(maxdate) values
((select max(date1) from table1)), -- goes in row1
((select max(date2) from table2)), -- goes in row2
.
.
.
((select max(date500) from table500));--goes in row500
is it possible that while insertion , order of inserting might get change ?.Eg when i will do
select maxdate from table_name limit 500;
i will get these
date1 date2 . . date253 date191 ...date500
Short answer:
No, not possible.
If you want to double check :
mysql> create table letest (f1 varchar(50), f2 varchar(50));
Query OK, 0 rows affected (0.00 sec)
mysql> insert into letest (f1,f2) values
( (SELECT SLEEP(5)), 'first'),
( (SELECT SLEEP(1)), 'second');
Query OK, 2 rows affected, 1 warning (6.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from letest;
+------+--------+
| f1 | f2 |
+------+--------+
| 0 | first |
| 0 | second |
+------+--------+
2 rows in set (0.00 sec)
mysql>
SLEEP(5) is the first row to be inserted after 5 seconds,
SLEEP(1) is the second row to be inserted after 5+1 seconds
that is why query takes 6 seconds.
The warning that you see is
mysql> show warnings;
+-------+------+-------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------+
| Note | 1592 | Statement may not be safe to log in statement format. |
+-------+------+-------------------------------------------------------+
1 row in set (0.00 sec)
This can affect you only if you are using a master-slave setup, because the replication binlog will not be safe. For more info on this http://dev.mysql.com/doc/refman/5.1/en/replication-rbr-safe-unsafe.html
Later edit: Please consider a comment if you find this answer not usefull.
Yes, very possible.
You should consider a database table unordered, and a SELECT statement without ORDER clause as well. Every DBMS can choose how to implement tables (often even depending on Storage Engine) and return the rows. Sure, many DBMS's happen to return your data in the order you inserted, but never rely on it.
The order of the retrieved data my depend on the execution plan, and may even be different when running the same query multiple times. Especially when only retrieving part of the data (TOP/LIMIT).
If you want to impose an order, add a field which orders your data. Yes, an autoincrement primary key will be enough in many cases. If you think you'll be wanting to change the order someday, add another field.
From my create table script, I've defined the hasMultipleColors field as a BIT:
hasMultipleColors BIT NOT NULL,
When running an INSERT, there are no warnings thrown for this or the other BIT fields, but selecting the rows shows that all BIT values are blank.
Manually trying to UPDATE these records from the command line gives odd effect - shows that the record was match and changed (if appropriate), but still always shows blank.
Server version: 5.5.24-0ubuntu0.12.04.1 (Ubuntu)
mysql> update pumps set hasMultipleColors = 1 where id = 1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0
mysql> select hasMultipleColors from pumps where id = 1;
+-------------------+
| hasMultipleColors |
+-------------------+
| |
+-------------------+
1 row in set (0.00 sec)
mysql> update pumps set hasMultipleColors = b'0' where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select hasMultipleColors from pumps where id = 1;
+-------------------+
| hasMultipleColors |
+-------------------+
| |
+-------------------+
1 row in set (0.00 sec)
Any thoughts?
You need to cast the bit field to an integer.
mysql> select hasMultipleColors+0 from pumps where id = 1;
This is because of a bug, see: http://bugs.mysql.com/bug.php?id=43670. The status says: Won't fix.
You can cast BIT field to unsigned.
SELECT CAST(hasMultipleColors AS UNSIGNED) AS hasMultipleColors
FROM pumps
WHERE id = 1
It will return 1 or 0 based on the value of hasMultipleColors.
You need to perform a conversion as bit 1 is not printable.
SELECT hasMultipleColors+0 from pumps where id = 1;
See more here:
http://dev.mysql.com/doc/refman/5.0/en/bit-field-literals.html
The actual reason for the effect you see, is that it's done right and as expected.
The bit field has bits and thus return bits, and trying to output a single bit as a character will show the character with the given bit-value – in this case a zero-width control character.
Some software may handle this automagically, but for command line MySQL you'll have to cast it as int in some way (e.g. by adding zero).
In languages like PHP the ordinal value of the character will give you the right value, using the ord() function (though to be really proper, it would have to be converted from decimal to binary string, to work for bit fields longer than one character).
EDIT:
Found a quite old source saying that it changed, so a MySQL upgrade might make everything work more as expected: http://gphemsley.wordpress.com/2010/02/08/php-mysql-and-the-bit-field-type/