r - Why am I getting warnings about closing unused RODBC handles? -

i use rodbc r , knitr reporting using various production databases. in of these reports i'm running multiplie queries against multiple databases.

each of queries carried out in function of form:

get.total.orders <- function(db.connex.string, start.date, end.date){     db.connex <- odbcdriverconnect(db.connex.string)     ord.qry <- sprintf("select ord_orderreference 'order.ref', ord_registerdate 'register.date' orders ord_registerdate >= '%s' , ord_registerdate < '%s'",                        start.date, end.date)     orders <- sqlquery(db.connex, ord.qry)     odbcclose(db.connex)     return(orders) } 

note odbc channel opened , closed in function, , single, simple query run between opening , closing of channel.

nonetheless, when run report more once (e.g. when developing report), receive warnings such following:

warning: closing unused rodbc handle 41 

the more times run report, higher number of handle reported in error becomes.

why, if i'm opening , closing channel in query function, being left open, 'unused' rodbc handles?

more importantly, how can avoid issue?

i avoid using on.exit:

get.total.orders <- function(db.connex.string, start.date, end.date){    db.connex <- odbcdriverconnect(db.connex.string)    on.exit(odbcclose(db.connex))  # <-----------------------   change here    ord.qry <- sprintf("select ord_orderreference 'order.ref',       ord_registerdate 'register.date'          orders          ord_registerdate >= '%s' , ord_registerdate < '%s'",          start.date, end.date)  orders <- sqlquery(db.connex, ord.qry)  return(orders)  } 

this way, connection closed if there error. see ?on.exit.

[edit]

the above presumes handle not closed because there error executing query. if query ok handle wasn't closed have no idea. odbcclose returns 0 if succeeded might check that.

[edit2]

as others have pointed out, nothing worry - on other hand, still interesting figure out why connection not closed if explicitly tell close. maybe matter of milliseconds , query not yet finished when result being assigned. doesn't make sense me if result assigned orders else there database? maybe there something. in case, 1 might try give more time eg.

#... orders <- sqlquery(db.connex, ord.qry) orders # or force(orders) - evaluate result once more sys.sleep(0.01)  # give 10 milliseconds orders # or return(orders) - return result # presuming on.exit before - odbcclose happen here } 

this sounds stupid wouldn't surprised if work.

another idea if using rstudio may phantom error messages happens example, when using plot non-existing graphical parameter fist time, , no errors on second time.

plot(1, bimbo=2)  # here warnings bimbo not graphical parameter plot(2)   # nothing wrong here rstudio replays previous warnings 

maybe similar happens db handlers -- if case, instructive see if same warnings in both rstudio , console (rgui or rterm in windows or running r in terminal in linux). of course applies if using rstudio.

and finally, might try posting on r-help brian ripley (one of authors of rodbc) there not here.

so see, don't have real answer , if takes effort figure out, recommend not worrying :)