Book: "Pentaho Kettle Solutions", Matt Casters, Roland Bouman, & Jos van Dongen, Wiley 2010 Book: "Pentaho Solutions", Roland Bouman & Jos van Dongen, Wiley 2009

Wednesday, September 06, 2006

Refactoring MySQL Cursors

Quite a while ago, I wrote a few articles on using MySQL cursors. This time, I will try and shed some light on why cursors are often unnecessary. To illustrate that, I will refactor a typical case of cursor usage.

The first part of this article explains why cursors are usually unnecessary. A few common problems with cursors are briefly discussed. Also, typical stored procedure pattern is described that uses a cursor, and a demonstration is given that shows how it can be refactored to an equivalent procedure that uses a single SQL statement instead.

In the second part of this article, the negative performance implications of using cursors are illustrated with a few benchmarks, and the cases where a cursor might be useful after all are briefly discussed.

Cursor trouble


Some time ago, I used to be quite active in some of the MySQL Forums, especially the ones that cover the features introduced in MySQL 5.0 (views, stored procedures, triggers and cursors).

After a little while it became quite obvious to me that most of the problems that were posted in the the cursor forum could be grouped into just a few classes.
The following lists them in ascending order according to the abundance (as perceived by me) of posts mentioning that particular type of problem.


Attempts to use unsupported features

In a minority of cases, people expected a feature that just isn't supported by MySQL. To name a few useful ones: cursor FOR loops, FOR UPDATE cursors and passing cursors to and from stored procedures as a parameter.

Nested Cursor loops

Nesting cursors is usually a combined problem of the following two problems. If you really need nested cursors, then you have to be extra careful with loop handling. However, usually, nesting cursors is entirely unnecessary and inappropriate. (Many, many nested cursor loops can and should be written as simple JOINs.)

Basic cursor loop handling problems

This really is quite a common problem. To be frank, there are quite a few things you need to think of if you want to do proper cursor looping in MySQL, and I think I can safely call it a fact that a lot of people run into trouble there.

Unnecessary usage of cursors

This is by far the most seen problem with cursors. In maybe as many as 80% of the cases where a cursor is used, it isn't needed at all. In quite a lot cases, the cursor can be written as a single SQL statement.



In the forum, I found myself explaining the same problem and the solution over and over again. So, I wrote little articles to illustrate basic loop handling (Want to write a CURSOR LOOP with MySQL?,
Why REPEAT and WHILE are usually not handy to handle MySQL CURSORs) and cursor nesting (Nesting MySQL Cursor Loops). These articles gave me a quick pointer to throw with in case I bumped into yet another thread requesting help regarding these types of problems.

On some occasions, I have briefly pointed out that cursors are often unnecessary.
This time, I'll try to elaborate on this a little further. My objective is to illustrate how a typical case of using cursors can be refactored into a single SQL statement that uses no cursor at all. We'll see how this allows us to discard about two thirds of the code and gain considerable performance at the same time.

Are you Sure you need a cursor?



In quite a few cases where I tried to help by explaining in what way the loop handling or the nesting was flawed, I pointed out that although the problem could be fixed, it was probably a better idea to do away with the cursor altogether. More often than not it was not too hard to think of a way to rewrite the entire cursor loop (including the body) as a single SQL statement.

However, it sometimes turned out to be quite hard to convince the person that originally wrote the cursor loop. In many cases, cursors are used by developers that are more familiar with a procedural or object-oriented language than with SQL.

I feel that a single SQL statement is nicer, more elegant. Usually, it involves less code, and often it is easier to understand. However, these are matters of aesthetics and preference, and thus do not count - not really. In my opinion, no matter how useful, creative, revolutionary or brilliant a piece of software is, it will never become a work of art (although it may demonstrate sublime craftmanship).

Luckily, there is an extra reason to avoid cursors in favour of single SQL statements. It's a good one too, and it will often convince the most stubborn procedurally oriented programmer. The extra reason is speed. I think it's safe to say that a single SQL statement will always outperform the equivalent cursor loop.

A nearly Real World Example


My nearly Real World Example is some sort of reservation database. The particular nature of the reservation is not really important. We can pretend that this concerns a car-rental shop, or maybe a hotel. The important thing is that customers can make reservations and that the reservation lists the reserved products.

A product has a default value for the price (value_prod), vat percentage (vat_prod) and discount percentage (discount_prod). The items that appear on a reservation have a reference to the reservation, and a reference to the product. Like a product, an item too has a price (unit_value), vat percentage (vat_value) and a discount percentage (discount). In addition, the item also stores the number of rental days (no_days), and a flag (disc_bool) that determines whether a discount will be charged. The products are stored in the product table, and the items are stored in the line_reserve table.

At first it might seem as if the unit_value, vat_value and discount columns in the line_reserve table are redundant, because these values could be derived from the related product table. However, this is not the case. The idea is that a reservation maybe made in advance. So, the vat and discount percentages as well as the price of the product could change after the reservation was made.

(Although it might be tempting to dicuss the database design, that is not the goal of this article. We won't discuss the validity of the database design here - it just serves as the background for a stored procedure I want't to describe.)

A procedure to update price, vat and discount for reserved items



The database also contains a UpdatePriceAndVatAndDiscountForReservationItems procedure. The procedure is meant to do the actual legwork of calculating and storing
the vat, discount and price values for all items that make up a reservation. The reason why I'm higlighting this procedure is because it contains a pattern of improper cursor handling, and I hope to demonstrate how that pattern can be recognized and rewritten.

(I'm deliberately not discussing any issues concerning coding style, such as identifier conventions. Also, I won't go into the detail concerning the chosen datatypes. Although improvements might be possible in this respect, this article tries to focus on the cursor loop)

Here's the code:

CREATE PROCEDURE `UpdatePriceAndVatAndDiscountForReservationItems`(
IN var_id_res INTEGER
)
BEGIN
-- the product record
DECLARE var_unit_val DOUBLE;
DECLARE var_vat DOUBLE;
DECLARE var_discount DOUBLE;

-- the line_reserve record
DECLARE var_id_line INTEGER;
DECLARE var_id_prod INTEGER;
DECLARE var_disc TINYINT;
DECLARE var_no_days INTEGER;

-- the calculated vat
DECLARE var_val_vat DOUBLE;

-- cursor loop book-keeping
DECLARE no_more_rows BOOLEAN;
DECLARE num INTEGER;

-- the line reserve curosr
DECLARE cur_res CURSOR FOR
SELECT id_line
, id_prod
, disc_bool
, no_days
FROM line_reserve
WHERE id_res = var_id_res;

-- more cursor loop book-keeping
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET no_more_rows = TRUE;

-- loop control
OPEN cur_res;

loop_cur_res: LOOP
FETCH cur_res
INTO var_id_line
, var_id_prod
, var_disc
, var_no_days;

IF no_more_rows THEN
CLOSE cur_res;
LEAVE loop_cur_res;
END IF;


-- get corresponding product
SELECT value_prod
, vat_prod
, discount_prod
INTO var_unit_val
, var_vat
, var_discount
FROM product
WHERE id_prod=var_id_prod;

-- calculate vat
SET var_val_vat =
var_unit_val
* ROUND((var_vat/100),2);

-- calculate discount
IF var_disc=1 THEN
SET var_discount =
((var_unit_val+var_val_vat)*var_no_days)
* ROUND((var_discount/100),2);
END IF;

-- update line_reserve with vat and discount
UPDATE line_reserve
SET unit_value = var_unit_val
, vat_value = var_val_vat
, discount = var_discount
WHERE id_line = var_id_line;

-- record the line number
SET num = num+1;
END LOOP loop_cur_res;

-- return line count
SELECT num as result;
END;


(I admit I'm guilty of including some comments and indentation to aid the readability of the code. So, now you know why I call it a nearly real world example.)

The skeleton


The procedure uses the var_id_res parameter to specify a particular reservation:

CREATE PROCEDURE `UpdatePriceAndVatAndDiscountForReservationItems`(
IN var_id_res INTEGER
)
...

This procedure parameter is used to control a cursor to select only items that correspond to the reservation passed by the parameter:

-- the line reserve curosr
DECLARE cur_res CURSOR FOR
SELECT id_line
, id_prod
, disc_bool
, no_days
FROM line_reserve
WHERE id_res = var_id_res;

The procedure loops through the cursor with an ordinary unstructured LOOP, fetching a record from the cursor for each iteration:

-- loop control
OPEN cur_res;
loop_cur_res: LOOP
FETCH cur_res
INTO var_id_line
, var_id_prod
, var_disc
, var_no_days;
IF no_more_rows THEN
CLOSE cur_res;
LEAVE loop_cur_res;
END IF;
...
-- record the line number
SET num = num+1;
END LOOP loop_cur_res;
-- return line count
SELECT num as result;

The loop is controlled with the no_more_rows flag. The value of the flag is switched inside a NOT FOUND handler:

DECLARE CONTINUE HANDLER FOR NOT FOUND
SET no_more_rows = TRUE;

This ensures the loop is terminated properly when the cursor is exhausted.

A little side-note. The end of the loop code contains explicit code to count the number of lines that are processed by the cursor: the num variable is incremented for each iteration of the loop, and the value is selected and returned to the client caller right after the end of the loop. However, it is flawed. the procedure always returns a NULL for the result column:

mysql> call UpdatePriceAndVatAndDiscountForReservationItems(76);
+--------+
| result |
+--------+
| NULL |
+--------+
1 row in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)

That's because of an easily overseen detail: the num variable was never initialized. Therefore, the value will remain NULL for evermore. The easiest way to prevent such trouble is to give such variables an explicit default value in their declaration:

DECLARE num INTEGER DEFAULT 0;

So far, we've seen nothing special.

The loop body


Now for the body of the loop. Each iteration essentially executes three separate things:

  1. Fetch data from the product that corresponds to the current reservation item

  2. Calculate the vat and discount using data from both the reservation item and the product

  3. Update the reservation item with the calculated values


Writing SELECT...INTO as a JOIN

For each record fetched from the cursor, the corresponding product record is retrieved with a SELECT...INTO statement. It uses the prod_id from the reservation item that was FETCH-ed in the top of the loop to identify exactly one corresponding record from the product table:

-- get corresponding product
SELECT value_prod
, vat_prod
, discount_prod
INTO var_unit_val
, var_vat
, var_discount
FROM product
WHERE id_prod=var_id_prod;

This is a good moment to realize a couple of things.

A SELECT...INTO statement can retrieve at most one record. In this case, it's probably probably quite safe to assume that it does, because the reservation item has a mandatory foreign key that references the primary key of the product table.

But suppose we would use this type of statement to retrieve data for an optional relationship. Well, val_id_prod could be NULL, and the SELECT...INTO statement not retrieve any record.

Ouch!

That would mean we're in big trouble: the loop is controlled using a NOT FOUND handler, and that's exaclty the handler that will be fired when this SELECT...INTO statement matches no records. Our loop would finish unintentionally, and most probably prematurely,

So, what can we do about it? Well, a lot of things. To name a few:

  • If we can be sure that a record will be matched when var_id_prod is NULL, we can wrap a IF...END IF around it.

  • We can wrap a BEGIN...END block around it, and declare a separate NOT FOUND handler there. Because that handler is nearer than the one that controls our loop, this will capture the condition, and the loop will never be left. The nice thing about this approach is that it also offers an opportunity to handle cases where the statement matches more than one record.

  • We can explicitly reset the loop control variable right before the FETCH statement. You will find more information on that solution here


However the real question is: what is a SELECT...INTO doing inside this loop? If we are sure it will match a record for each record fetched for the loop, why not rewrite the cursor to a join?

DECLARE cur_res CURSOR FOR
SELECT r.id_line
, r.disc_bool
, r.no_days
, p.value_prod
, p.vat_prod
, p.discount_prod
FROM line_reserve r
JOIN product p
ON r.id_prod = p.id_prod
WHERE id_res = var_id_res;


So, this is a pattern we can regonize, and avoid. Any SELECT...INTO that is executed for each iteration of a cursor loop, can be rewritten by joining it to the SELECT-statement of the cursor. The INTO fields are simply added to the
SELECT-list of the cursor query. In this particular case, we can also remove the id_prod column from the SELECT-list. It was only used to retrieve the product data, and this is now already solved.

We should keep in the back of our minds that when we change the SELECT-list we shoud also change the FETCH, or else there will be a mismatch in the number of expressions in the SELECT-list and the variables list used in the FETCH statement.

It is important to realize that this will not result in extra iterations, at least, not if the SELECT...INTO was a right choice in the first place.

Writing SET assignments as part of SELECT

The loop body continues by calculating the vat and discount values:

-- calculate vat
SET var_val_vat =
var_unit_val
* ROUND((var_vat/100),2);

-- calculate discount
IF var_disc=1 THEN
SET var_discount =
((var_unit_val+var_val_vat)*var_no_days)
* ROUND((var_discount/100),2);
END IF;

If we settle for using a JOIN in the cursor query instead of a SELECT...INTO inside the loop body, we might just as well add the expressions that calculate the vat and the discount to the SELECT-list:

DECLARE cur_res CURSOR FOR
SELECT r.id_line
, p.value_prod
* ROUND((p.vat_prod/100),2) var_val_vat
, if( r.disc_bool
, ( p.value_prod
* round(p.vat_prod/100,2)
+ p.value_prod
)
* r.no_days
* round(p.discount_prod/100,2)
, NULL
) var_discount
FROM line_reserve r
JOIN product p
ON r.id_prod = p.id_prod
WHERE id_res = var_id_res;

Again, we can discard columns we don't need anymore, and we must modify the FETCH-statement accordingly.

It seem trivial, but for completeless, we should note that this is a pattern too: any expressions that appear on the right-hand side of the assignment operator in a SET statement, can and probably should be written as an expression in the SELECT-list, and the actual assignment to variables is then handled in the FETCH-statement.

In the original example, the var_discount variable is conditionally set using an IF..THEN statement. If the IF..THEN statement only contains SET statements, then all of the corresponding expressions on the right hand side of the assignment operator can be written using the IF function. This can be useful if the expression is expensive or if it performs some kind of side-effect.
Rewriting the single row UPDATE

The final action that is performed inside the cursor loop is an UPDATE of the line_reserve table. In order to store the calculated vat and discount data, only the record that corresponds to the record fetched by the cursor is updated:

UPDATE line_reserve
SET unit_value = var_unit_val
, vat_value = var_val_vat
, discount = var_discount
WHERE id_line = var_id_line;

(There are rdbms-products that support a FOR UPDATE clause in the cursor declaration. This lets you modify the values in the cursor record immediately. MySQL does not support this feature.)

We should realize that UPDATE is perfectly capable of updating multiple records. In fact, any UPDATE statement that does not include a WHERE-clause, will update all the records in the table. To limit the update to only those line_reserve records that corrsepond to a reservation, we will need to add a proper WHERE-clause.
In fact, we know exactly which WHERE-clause we need, because we used it to write the cursor declaration:

UPDATE line_reserve
SET unit_value = ?
, vat_value = ?
, discount = ?
WHERE id_res = var_id_res;

Of course, we cannot just use the original variables for the column assignments, because the value of these variables potentially varies for each record touched by the UPDATE-statement. Thinking of an expression that calculates them directly as part of the UPDATE-statement poses a slight problem, because we have seen that the calculation of vat and discount depend on values from both the line_reserve record, as well as the corresponding product record.

So, somehow, we need to find a way for the corresponding product records to contribute their values to the UPDATE statement. One way of solving this problem is using subqueries:

UPDATE line_reserve r
SET unit_value = (
select p.value_prod
from product p
where p.id_prod = r.id_prod
)
, vat_value = (
select p.value_prod
* ROUND((p.vat_prod/100),2)
from product p
where p.id_prod = r.id_prod
)
, discount = (
select if( r.disc_bool
, ( p.value_prod
* round(p.vat_prod/100,2)
+ p.value_prod
)
* r.no_days
* round(p.discount_prod/100,2)
, p.discount
)
from product p
where p.id_prod = r.id_prod
)
WHERE id_res = var_id_res;

Before explaining these subqueries, please take a moment to realize that this single statement does all the work the cursor loop used to do. So, this proves that the cursor loop really was unnecessary in the sense that we can perform the same task using a single SQL statement.

The subqueries are the parenthesis enclosed SELECT expressions appearing on the right-hand side of the assignment operator in the SET-clause. In order for a subquery to be used like this, the subquery needs to be scalar: they must select a single row, with a single column. Because of this special form, the 'resultset' can be interpreted as a scalar: a simple, singular value-expression.

Note that these three subqueries differ only as far as the SELECT-list is concerned. They all select one single row from the product table that corresponds to the current line_reserve record. To select only the corresponding product record, the subquery is bound to the main query using an appropriate WHERE-clause.

(It is no coincidence that the WHERE-clause used here is identical to the one used in the SELECT...INTO statement in the original code.)

Subqueries that use expressions from the surrounding query to limit the resultset are said to correlated.

Some people might feel uncomfortable about the repetition of code. Three times, essentially similar queries are performed. Even if there would be some kind of optimization that accounts for any performance issues that might rise from that, it is still objectionable that we must repeatedly write part of the code.

Some rdbms-products allow a a variation of this kind of syntax to address this issue:

UPDATE line_reserve r
SET (
unit_value
, vat_value
, discount
) = (
select value_prod
, p.value_prod
* ROUND((p.vat_prod/100),2)
, if( r.disc_bool
, ( p.value_prod
* round(p.vat_prod/100,2)
+ p.value_prod
)
* r.no_days
* round(p.discount_prod/100,2)
, p.discount
)
from product p
where p.id_prod = r.id_prod
)
WHERE id_res = var_id_res;

So, the update is performed by assigning a complete record. This kind of syntax is not supported by MySQL. But as we shall see, there is a much better way to handle this. MySQL and a few other rdbms products support the JOIN syntax as part of an UPDATE statement.

We already discussed how a JOIN could be used to fetch product data corresponding to the line_reserve records in the cursor declaration. Now all we have to do is to modify that statement to an update statement:

UPDATE line_reserve r
INNER JOIN product p
ON r.id_prod = p.id_prod
SET r.unit_value = p.value_prod
, r.vat_value = p.value_prod * round(p.vat_prod/100,2)
, r.discount = if( r.disc_bool
, ( p.value_prod
* round(p.vat_prod/100,2)
+ p.value_prod
)
* r.no_days
* round(p.discount_prod/100,2)
, r.discount
)
WHERE r.id_res = var_id_res;

So in this case, the JOIN is performed, and any assignments that are applied in the SET-clause are somehow pushed through to the underlying table. At first, this might seem a bit awkward, but it is actually quite elegant once you get used to it. It is generally faster too than a solution with subqueries, and you can actually use this to modify multiple tables at once, something that can occasionally be quite useful.
Returning the rowcount

There is only one thing in the original that we did not account for yet. The original procedure returned the number of records that were actually touched by the procedure. For the single statement solution, we can use the built-in ROW_COUNT() function in MySQL. This function returns the number of records that were affected by the last DELETE, INSERT or UPDATE statement.

Finally


We managed to rewrite the orignal 68 line stored procedure to this mere 23 lines of code:

CREATE PROCEDURE
`sp_UpdateLineReserveVatAndDiscount`(
IN p_id_res INTEGER
)
BEGIN
UPDATE line_reserve l
INNER JOIN product p
ON l.id_prod = p.id_prod
SET l.unit_value = p.value_prod
, l.vat_value = p.value_prod * round(p.vat_prod/100,2)
, l.discount = if( l.disc_bool
, ( p.value_prod
* round(p.vat_prod/100,2)
+ p.value_prod
)
* l.no_days
* round(p.discount_prod/100,2)
, l.discount
)
WHERE r.id_res = p_id_res;

SELECT ROW_COUNT();
END;

This clearly illustrates that the cursor was not necessary in this case. To me, it also illustrates how extraordinarily powerful pure SQL is, because there must be some kind of mechanism that actually does iterate through all the records to perform the join and update actions on a low-level. The SQL language allows us to express the actual data manipulation
without requiring that we specify anything about the workflow that is needed to retrieve and wade through the individual records. This is certainly one of the reasons why I like this refactored solution better.

What about speed?


Is there anything we can say about how fast these different procedures run? Sure!

I created a simple benchmarking procedures to repeatedly execute a modified version of the two procedures. I modified the procedures by discarding the final SELECT statement that returns the number of affected records. For performing the benchmarks, I found it impractical to have the resultset returned and because the purpose was primarily to show the difference between the cursor and the single statement, I gathered that it was safe to leave it out.

I also generated series of data: a series of reservations with 1,10,100,250,500,750 and 1000 of corresponding line_reserve records, all of them referencing just one product record. There was only a very small (5) amount of product records in the table as a whole at this point.

As far as the rest of environment is concerned: the benchmark was performed on a lenovo 3000 n100 1.8Gb/1024 Mb (centrino Duo) notebook with Ubuntu linux and MySQL 5.1.11-beta. Executing the procedures was done using the mysql commandline utility. The tables are InnoDB tables, and there were proper indexes on the relevant columns. MySQL configuration is completely standard, but autocommit was turned off during a single procedure execution.

I then had my benchmarking procedures execute the procedure 10000 times for each of the series. Running the benchmarking procedure was repeated 2 to 5 times. For each run of the benchmarking procedure, the time needed to complete the benchmarking procedure was recorded, and averaged.

Then, more data was generated for the product table as well so that it contained 1000 records. The line_reserve table was updated, randomizing the product references.

So, to summarize: there are four different sets of measurements:

  • the original procedure using the cursor

    • few product records, and only one used product (cursor1)

    • randomized usage acros 1000 product records (cursor1000)



  • the refactored procedure using a single statement

    • few product records, and only one used product (statement1)

    • randomized usage acros 1000 product records (statement1000)




For each of these, a series of reservations with 1,10,100,250,500,750 and 1000 records was tested by executing the procedure 10000 times. The time required to do that was recorded. Elapsed time was then overaged over 2 to 5 runs of 10000 cycles.

The results are shown in this chart:

An important fact is that the single statement solution was faster than the cursor solution under all tested circumstances. Depending upon the exact situation, the single statement solution was at least 1.44 times as fast as the cursor solution, and at most 8.25 times as fast. The message is very clear if speed is all you care about: avoid cursors as much as you can in favour of single SQL statements.

In nearly all cases, the increase in the number of processed line_reserve records appears to have a linear effect on the increase in execution time. The one exception is the measurement of the cursor method for the 500 line_reserve records with randomized references to the available 1000 products. In that particular case, the execution time takes a drop and is only a little higher than the corresponding figure for 100 line_reserve records, but considerably lower than the corresponding figure for 250 line_reserve records. At present, it is unclear what could be causing this peculiar effect.

The differences between the two methods became bigger when more line_reserve records were involved. This seems to indicate that the overhead of setting up and running a cursor loop is just a little bigger than executing a single statement, whereas the direct processing of the rows inside a single SQL statement is considerable faster than performing row-by-row operations explicitly with a cursor. This is not very surprising when we realize that opening the cursor loop requires the execution of an SQL statement in addition to processing the resultset row by row.

Surprisingly, the addition of extra product records sped up execution time for both the cursor as well as the single-statement solution. This was unexpected. It was expected that the execution time of the single statement solution would remain nearly the same, but that the execution time of the cursor solution would increase, possibly quite a great deal. This expectation was based on the idea that only a little increase in the time needed to find a single product record would have a quite large effect on the cursor loop execution time, because it fetches a single product record in each iteration of the loop. However, that assumption is clearly not supported by these results.

Possibly this is caused by the total number of records in the product table:
One can imagine that in the initial situation (5 product records) a full table scan was performed to find the correct record,whereas 1000 records in the product table would result in using an index for the data access, speeding up the query. However, this would mean that a full table scan, even for 5 records, is an extremely expensive operation - something I cannot believe to be true,.

So, why use cursors at all?


The question remains why you would use cursors at all. In what cases do we really need one, or what benefit could a cursor have.

Well, I already pointed out a few reasons in one of my earlier articles. There are some things you just cannot do in a single SQL statement. For example, suppose you want execute SQL statements dynamically, and the statements are generated by another SQL statement. There is no way to do that without a cursor, so in this case you really need one. (If this example sounds artificial, check out this snippet on MySQLForge).

How about your story


If you have an explanation for the exact figures seen in the benchmark, or if you have any ideas on how to improve the benchmark, please post a comment. Also, if you have a good, convincing example that illustrates why you really do need cursors sometimes, please let me know. Just leave a comment on the blog.

7 comments:

Anonymous said...

Damn.
I think that there are people, lile me :-) , that use cursors because they have viDamn.
I think that there are people, like me :-), that use cursors because they have vices from old school programming. In your testimony, you showed me how simple and fast a problem can be solved. Sure that “…there are many ways to solve the same problem…” but why use the wrong one? I’ll be looking forward to see the next article in your blog. This one taught me a lot about simple SQL, and how we can “mix” things inside it.
Thanks a lot and keep the good work.

Jorge Ferreira
ces from old school programming. In your testemony

Matt Casters said...

Hi Roland,

Congratulations on another thourough article. From an application developers viewpoint I couldn't agree with you more.

It's a different thing when you look at MIS usage...

To be honest, it has taken me 3 YEARS to figure out how to enable server side cursor emulation on MySQL just to prevent my client
(Kettle) of running out of memory.

In situations where you have a huge amount of records that need to pass to the client, you have no other choice but to use cursors.
Otherwise, instant crashing when running out of memory is your fate :-)

Mmm, perhaps I should blog about that too ;-)

I'm having mixed feelings about reading your article. On the one hand it's exellent and really relevant. On the other hand... Well, it just reminds me too much of the things that Oracle Warehouse Builder does: it tries to compress complete transformations with dozens of steps in a single Select/Insert/Update statement ... without using cursors.

The thing about that is that in theory this works very fast and nice, in practice however, IMHO, it's a debugging and maintenance Nightmare on Elm Street. You expect Freddy Cruger to pop up every time to slice you up because of a problem with either: (pick your favorite)
[ ] weird bug
[ ] a database limit
[ ] a behaviour that not quite goes as you planned
[ ] a optimisation plan that goes awry
[ ] all of the above

I'm in no way saying that this applies to MySQL, but I felt shivers run down my spine when I saw those update subselect statements. It just looked too similar.

Anyway, just offering an alternate view. :-)

All the best,

Matt

Roland Bouman said...

Hi Matt,

thanks for offering your take on it.

I actually did use cursors in Oracle too in a partiuclar case. The cursor was used there to prevent a transaction from growing too big or too long-lasting. A cursor was used to spread the load, committing once every X records.

Your comment made me think about that again, and it got me wondering. When we stick to the example described in the blog, I think there are two things that could go wrong when dealing with extreme amounts of data:

1) the construction of the resultset could run out of memory.
2) the actual update could run out of memory, or might take too long and time out.

Now when I used a cursor for loading in Oracle, I recall that a direct, single statement did fail (with the dreaded "could not allocate x bytes" or similar message), whereas the cursor solution did not, suggesting that the direct statement failed of the second reason.

Now assuming that that was indeed the underlying reason, it puzzles me if it is possible wheter just opening a cursor could fail. I reckon it could if the entire resultset would be constructed as part of opening the cursor.

Does anyone know anything about this, either in Oracle or in MySQL?

I really am interested in your method to emulate a MySQL server side cursor. So, please do blog about that, I think that would be very valuable info for others.

When writing the article I actually did think of a case where you'd want to 'partition' as it were a huge INSERT or UPDATE using a cursor, and I decided that it would probably be feasible using single statements and the MySQL proprietary LIMIT clause, so I am really curious to see whether your solution is along those lines.

DataSpewer said...

I'll blog about it in a couple of weeks, we're doing a MySQL Webinar next week and I still need to prepare a lot of things ;-)

As for the cursors... the real answer is of-course: "it depends" on when you should use those. But I agree with your sentiment that it should not always be the first thing to go for when writing applications. :-)

Obviously, avoiding cursors is something that's going to put more stress on your RDBMS. About the question : do you want to have a 1 second load spike or a 5 seconds modest load? I don't think it's that simple. I have seen plenty of cases where you would get a 5 seconds spike using cursors. Re-writing like you said can indeed save you a lot of time.

slowlearner said...

I much appreciate your explanation regarding the unneeded use of cursors. I learned more here than a whole semester in my DBMgmt class... Too bad your efforts to redirect people to this article don't work out so well.

The benchmarking graph was very interesting.
Many thanks.

Anonymous said...

Hi,
I am working in a ERP Human resource management application which handling the employee attendance process.

In that I created the stored procedure "sp_leave_processing" for processing the leave taken by employees,procedure input parameter is the attendance date.

Here i process the procedure with cursor.Cursor SQL statement is select active employees from employee table and process the leave in cursor loop.

In this cursor loop i am checking whether the employee is present or absent for given date, there is another table contain the employee present entries for each day.If employee absent for that day i have to check is there any prior leave request available if yes process the leave count and not present then process the without pay leave.

My query is how to avoid the cursor for this process is there any way to do this attendance process.

Thanks in advance.

Roland Bouman said...

Anonymous,

I can't tell if you can replace the cursor with a regular SQL statement unless I see the code. In particular, it is unclear what you mean by "process the leave count" and "process the leave without pay count".

It is however possible to write the dataset as a single query. You would get:

SELECT e.id
, IF( a.id IS NULL
AND l.id IS NOT NULL
, 1, 0) leave_count
, IF( a.id IS NULL
AND l.id IS NULL
, 1, 0) leave_without_pay_count
FROM active_employees e
LEFT JOIN employee_attendance a
ON e.id = a.employee_id
AND param_attendance_date =
a.attendance_date
LEFT JOIN leave_request l
ON e.id = l.employee_id
AND param_attendance_date BETWEEN l.leave_from AND l.leave_to