sql-execution-plan

I'm analysing Oracle execution plans and found an astonishing fact. Check out this query. The hint is just to display that I have an index and I'd expect Oracle to use it for range scans: // execute_at is of type DATE. PreparedStatement stmt = connection.prepareStatement( "SELECT /*+ index(my_table my_index) */ * " + "FROM my_table " + "WHERE execute_at > ? AND execute_at < ?"); These two bindings result in entirely different behaviour (to exclude bind variable peeking issues, I actually enforced two hard-parses): // 1. with timestamps stmt.setTimestamp(1, start); stmt.setTimestamp(2, end); //

One of my stored procedures was taking too long execute. Taking a look at query execution plan I was able to locate the operation taking too long. It was a nested loop physical operator that had outer table (65991 rows) and inner table (19223 rows). On the nested loop it showed estimated rows = 1,268,544,993 (multiplying 65991 by 19223) as below: I read a few articles on physical operators used for joins and got a bit confused whether nested loop or hash match would have been better for this case. From what i could gather: Hash Match - is used by optimizer when no useful indexes are available,

I have a Sqlite database which I want to check the indexes are correct. MS SQL Analyser is great at breaking down the query execution and utilised indexes. Is there a similar tool for Sqlite? I know of no pretty graphical tools, but all of the information you seek is available from the EXPLAIN keyword. Consider this database: sqlite> create table users (name, email); sqlite> create index user_names on users (name); A query predicated on email will not use an index: sqlite> explain select * from users where email='foo'; 0|Trace|0|0|0||00| 1|String8|0|1|0|foo|00| 2|Goto|0|13|0||00| 3|OpenRead|0

If I run the following SQL query SELECT * FROM A LEFT JOIN B ON A.foo=B.foo WHERE A.date = "Yesterday" Does the WHERE statement get evaluated before or after the JOIN ? If after, what would be a better way to write this statement so that returns only rows in A from "Yesterday" are joined to B ? KM. It depends on the database. On SQL Server, run: SET SHOWPLAN_ALL ON then run the query, you will get an idea of what happens when it runs. Your idea of "evaluation" is not correct as SQL is a declarative language. BTW you can see the query execution plan. In MySQL prefix your query with keyword

I have been doing a lot of reading up on execution plans and the problems of dynamic parameters in stored procedures. I know the suggested solutions for this. My question, though, is everything I have read indicated that SQL Server caches the execution plan for stored procedures. No mention is made of Table-value functions. I assume it does so for Views (out of interest). Does it recompile each time a Table-value function is called? When is it best to use a Table-value function as opposed to a stored procedure? gbn An inline table valued function (TVF) is like a macro: it's expanded into the

What is the meaning of Select tables optimized away in MySQL Explain plan? explain select count(comment_count) from wp_posts; +----+-------------+---------------------------+-----------------------------+ | id | select_type | table,type,possible_keys, | Extra | | | | key,key_len,ref,rows | | +----+-------------+---------------------------+-----------------------------+ | 1 | SIMPLE | all NULLs | Select tables optimized away| +----+-------------+---------------------------+-----------------------------+ 1 row in set (0.00 sec) Note: explain plan output edited for legibility. It means you have

Im wondering how I can fetch the explain plan using Java. Reason I need this is because we have a framework where special users can craft reports. These reports sometimes build huge queries in which we want to explain on the fly and store the cost of. This way we can analyse the high cost queries later on and optimize. Example code which gives me illegal column exception: ResultSet rs = null; try { oracle = ConnectionManager.getConnection(ConnectionManager.Test); pstmt = oracle.prepareStatement("begin execute immediate 'explain plan for SELECT 1 from Dual'; end;"); rs = pstmt.executeQuery();