通过matlab使用mysql时找不到空闲句柄

Question

I'm accessing a mysql database via matlab on a fedora13 box.

Problem: I can successfully open a connection, read from tables and close a connection to a database. I can repeat this for multiple different tables. However, after some time (2-5 queries) I hit a wall and the next mysql open call throws the exception "Can't find free handle". The only way to reconnect to any database after this is to restart matlab. I've checked variables in memory (via whos) and checked for processes running on the database (show processlist;) but have not been able to find whats blocking my attempts to open connections.

Looking at mysql.cpp , it seems to be the case that MAXCONN might be too low, or my mysql close might not be handled correctly, but I have already eliminated those possibilities.

I would like to be able to open/close connections as my application needs without having to restart matlab in between to free database handles. Could someone offer some clarity as to how this process works?

Answer 1

maybe consider writing a function that holds the connection and only opens a new connection when the old one has gone away. That way you can reuse it.

In outline it looks

function conn = hold_connection_a
persistent local_connection
try
  test_connection(local_connection);
catch
  local_connection=establish_connection;
end
conn = local_connection;

establish connection then should be the function you use to connect and return your handle. test_connection should send a simple query - like SELECT 1 and produce an error if it fails.

By the way - if you use more than one databases it would make sense to either have multiple of this hold_connections or make sure your statements always contain the database name.

Unrelated: personally I use mYm since I find it quicker.

Answer 2

Use close(conn) to free the connection made by database(...) when you don't need it anymore. Changing MAXCONN to a higher value will just address the symptom, not the cause.

Answer 3

You can see the connections with

mysql

with no parameters.

mysql('close')

must solve such a problem.