I will get some text from another question here:
The PreparedStatement is a slightly more powerful version of a Statement, and should always be at least as quick and easy to handle as a Statement.
The Prepared Statement may be parametrized
Most relational databases handles a JDBC / SQL query in four steps:
Parse the incoming SQL query
Compile the SQL query
Plan/optimize the data acquisition path
Execute the optimized query / acquire and return data
A Statement will always proceed through the four steps above for each SQL query sent to the database. A Prepared Statement pre-executes steps (1) - (3) in the execution process above. Thus, when creating a Prepared Statement some pre-optimization is performed immediately. The effect is to lessen the load on the database engine at execution time.
Now here is my question:
If I use hundreds or thousands of Statement, will it be cause performance problems in database? (I don't mean that they will perform slower because of more jobs to do every time). Will all those statements be cached in database or they will be lost in space as soon as they are executed?
Since there is no restictions on using prepared statements, you should work carefully with them.
As you said you need hundreds of prepaired, think twice may be you are using it wrong.
The pattern it should be used is having an application that doing a haevy inserts/updates/select hundred or thousand times a second which only differs in variables. So in real world it would be like, connecting, creating session, sending statement, and sending bunch of variables to that statement.
But if your plan is to create prepared on each single operations, it's just better to use common queries.
On your questions:
Hundreds of statements will not kill mysql or drive you to performance degradation
The prepared are stored in memory while client session is up and running. As soon as you close session the prepared die.
To be sure you need it:
Your app able to execute statements fast so you get speed value of using them
Your query will not have a variable number of arguments, otherwise you can kill you app by creating objects and storing in memory on every statement
Related
I'm running into a issue with a spring boot application, in which I am getting the below error,
(conn=1126) Can't create more than max_prepared_stmt_count statements (current value: 16382)
Which seems to be hitting the ceiling of max_prepared_stmt_count in mysql. Increasing it as much as we want could be problematic as well as it could result in OOM killer issues like this.
I'm exploring if there are any ways to limit the creation of PreparedStatements in Spring boot.
One possible option that I can think of,
Avoiding lazy loading whenever possible would force Hibernate to fetch the data with lesser number of prepared statements thus avoiding the problem.
Cache the PreparedStatements created by hibernate.
If anyone solved this problem or with a deeper insight, please share your wisdom on solving this.
MySQL has no problem with many prepared statements if they are over time. Obviously some MySQL databases stay up and running for months at a time, serving prepared statements. The total count of prepared statements over those months can be limitless.
The problem is how many prepared statements are cached in the MySQL Server at any given moment.
It has long been a source of trouble for MySQL Server that it allocates a small amount of RAM in the server for each prepared statement, to store the compiled version of the prepared statement and allow it to be executed subsequently. But the server doesn't know if the client will execute it again, so it has to keep that memory allocation indefinitely. The server depends on the client to explicitly deallocate the prepared statement. This can become a problem if the client neglects to "close" its prepared statements. They become long-lived, and eventually all those accumulated data structures take too much RAM in the MySQL Server.
So the variable really should be named max_prepared_stmts_allocated, not max_prepared_stmt_count. Likewise the status variable Prepared_stmt_count should be Prepared_stmts_open or something like that.
To fix this in your case, I would make sure in the client code that you deallocate prepared statements promptly when you have no more need of them.
If you open a prepared statement using a try-with-resources block, it should automatically be closed at the end of the block.
Otherwise you should call stmt.close() explicitly when you're done with it.
We are currently doing a lot of small queries. We execute a query, read the results, and then execute the next one. Since network requests cost a lot of time, this ping-ponging gets slow very fast.
This is why we want to do multiple queries at once, sending all data that the SQL server must know to it, and only retrieving one result (consisting of multiple result sets).
We found that Qt 5.14.1's QSqlQuery has the nextResult() function, but in the documentation (link) it says:
Some databases may execute all statements at once while others may delay the execution until the result set is actually accessed, [...].
MY QUESTION:
So, does MySql Server 8.0 delay the execution until the result set is actually accessed? If this is the case, then we still have a ping-pong for every query right? Which would be very slow still.
P.S. Our current solution to just have 1 ping-pong is to union different result sets (resulting in kind of a block diagonal matrix) with lots and lots of null values), and this question is meant to find a better way to do this.
My situation:
MySQL 5.5, but possible to migrate to 5.7
Legacy app is executing single MySQL query to get some data (1-10 rows, 20 columns)
Query can be modified via application configuration
Query is very complex SELECT with multiple JOINS and conditions, it's about 20KB of code
Query is well profiled, index usage fine-tuned, I spent much time on this and se no room for improvement without splitting to smaller queries
With traditional app I would split this large query to several smaller and use caching to avoid many JOINS, but my legacy app does not allow to do that. I can use only one query to return results
My plan to improve performance is:
Reduce parsing time. Parsing 20KB of SQL on every request, while only parameters values are changed seems ineffective
I'd like to turn this query into prepared statement and only fill placeholders with data
Query will be parsed once and executed multiple times, should be much faster
Problems/questions:
First of all: does above solution make sense?
MySQL prepared statements seem to be session related. I can't use that since I cannot execute any additional code ("init code") to create statements for each session
Other solution I see is to use prepared statement generated inside procedure or function. But examples I saw rely on dynamically generating queries using CONCAT() and making prepared statement executed locally inside of procedure. It seems that this kind of statements will be prepared every procedure call, so it will not save any processing time
Is there any way to declare server-wide and not session related prepared statement in MySQL? So they will survive application restart and server restart?
If not, is it possible to cache prepared statements declared in functions/procedures?
I think the following will achieve your goal...
Put the monster in a Stored Routine.
Arrange to always execute that Stored Routine from the same connection. (This may involve restructuring your client and/or inserting a "web service" in the middle.)
The logic here is that Stored Routines are compiled once per connection. I don't know whether that includes caching the "prepare". Nor do I know whether you should leave the query naked, or artificially prepare & execute.
Suggest you try some timings, plus try some profiling. The latter may give you clues into what I am uncertain about.
I have a routine in MySQL that is very long and has multiple SELECT, INSERT, and UPDATE statements in it with some IFs and REPEATs. It's been running fine until lately, where it's hanging an taking over 20 seconds to complete (which is unacceptable considering it used to take 1 second or so).
What is the quickest and easiest way for me to find out where in the routine the bottleneck is coming from? Basically the routine is getting stopped up and some point... how can I find out where that is without breaking apart the routine and testing one-by-one each section?
If you use Percona Server (a free distribution of MySQL with many enhancements), you can make the slow-query log record times for individual queries, using the log_slow_sp_statements configuration variable. See http://www.percona.com/doc/percona-server/5.5/diagnostics/slow_extended_55.html
If you're using stock MySQL, you can add statements in the stored procedure to set a series of session variables to the value returned by the SYSDATE() function. Use a different session variable at different points in the SP. Then after you run the SP in a test execution, you can inspect the values of these session variables to see what section of the SP took the longest.
To analyze the query can see the execution plan of the same. It is not always an easy task but with a bit of reading will find the solution. I leave some useful links
http://dev.mysql.com/doc/refman/5.5/en/execution-plan-information.html
http://dev.mysql.com/doc/refman/5.0/en/explain.html
http://dev.mysql.com/doc/refman/5.0/en/using-explain.html
http://www.lornajane.net/posts/2011/explaining-mysqls-explain
I made a few tests, first I tested mysql prepared statement with $pdo->prepare() and $insert_sth->execute() for 10k inserts (with named parameters if it matters), and it took 301s.
After that I made simple insert queries and inserting each time for the same 10k inserts too and it took 303s.
So I would like to know: does prepared statements really give performance benefits? Because my tests didn't show it , or I have to optimize my prepared statements version in order for them to be they faster?
I can give my source code if it's needed.
I prefer prepared statements in terms of security rather than performance (not sure if it is faster) for example to avoid sql injection.
INSERTs are most likely IO-bound, since they're generally not very complex in terms of SQL - just a list of columns and data to put in them. Thus, what you use to perform the queries isn't as significant in the run time as the amount of data that you're stuffing into the database, how fast you can get the data to the DB server, and how fast the DB server can store it.