What is wrong with Cursors?

What's wrong with cursors is that they are often abused, both in Oracle and in MS SQL.

Cursor are for keeping a stable resultset which you can retrieve row-by-row. They are implicitly created when your query is run, and closed when it's finished.

Of course keeping such a resultset requires some resources: locks, latches, memory, even disk space.

The faster these resources are freed, the better.

Keeping a cursor open is like keeping a fridge door open

You don't do it for hours without necessity, but it does not mean you should never open your fridge.

That means that:

• You don't get your results row-by-row and sum them: you call the SQL's SUM instead.
• You don't execute whole query and get the first results from the cursor: you append a rownum <= 10 condition to your query

, etc.

As for Oracle, processing your cursors inside a procedure requires infamous SQL/PLSQL context switch which happens every time you get a result of an SQL query out of the cursor.

It involves passing large amounts of data between threads and synchronizing the threads.

This is one of the most irritating things in Oracle.

One of the less evident consequences of that behaviour is that triggers in Oracle should be avoided if possible.

Creating a trigger and calling a DML function is equal to opening the cursor selecting the updated rows and calling the trigger code for each row of this cursor.

Mere existence of the trigger (even the empty trigger) may slow down a DML operation 10 times or more.

A test script on 10g:

SQL> CREATE TABLE trigger_test (id INT NOT NULL)  2  /Table createdExecuted in 0,031 secondsSQL> INSERT  2  INTO   trigger_test  3  SELECT level  4  FROM   dual  5  CONNECT BY  6     level <= 1000000  7  /1000000 rows insertedExecuted in 1,469 secondsSQL> COMMIT  2  /Commit completeExecuted in 0 secondsSQL> TRUNCATE TABLE trigger_test  2  /Table truncatedExecuted in 3 secondsSQL> CREATE TRIGGER trg_test_ai  2  AFTER INSERT  3  ON trigger_test  4  FOR EACH ROW  5  BEGIN  6     NULL;  7  END;  8  /Trigger createdExecuted in 0,094 secondsSQL> INSERT  2  INTO   trigger_test  3  SELECT level  4  FROM   dual  5  CONNECT BY  6     level <= 1000000  7  /1000000 rows insertedExecuted in 17,578 seconds

1.47 seconds without a trigger, 17.57 seconds with an empty trigger doing nothing.

From MSDN:Cursor Implementations

Using a cursor is less efficient than using a default result set. In a default result set the only packet sent from the client to the server is the packet containing the statement to execute. When using a server cursor, each FETCH statement must be sent from the client to the server, where it must be parsed and compiled into an execution plan.

If a Transact-SQL statement will return a relatively small result set that can be cached in the memory available to the client application, and you know before executing the statement that you must retrieve the entire result set, use a default result set. Use server cursors only when cursor operations are required to support the functionality of the application, or when only part of the result set is likely to be retrieved.

I'm not an Oracle DBA, so I can't really speak to how the implementations are different. However, from a programming standpoint, set based operations are almost always faster than processing results in a cursor.

I have always been told that cursors where evil, but always by MS SQL Server gurus, because of it's bad performance. Regarding Oracle's PL/SQL I found this saying when to use cursors:

Not using cursors results in repeated parses. If bind variables are not used, then there is hard parsing of all SQL statements. This has an order of magnitude impact in performance, and it is totally unscalable. Use cursors with bind variables that open the cursor and execute it many times. Be suspicious of applications generating dynamic SQL.

As cursors are implicitly created on every operation, it doesn't seem so performance-punishing to use them when needed :)

Remember that Oracle's implementation is closer to Postgres than to Sybase (Genesis of MS SQL Server), so performance will be different for each on different tasks.If you can, avoid the hustle of tweak for performance on systems that can swap able back-ends, go for least common denominator if you need to work with both. /tangential_topic