On Mon, Mar 13, 2017 at 2:04 PM, Ryan Culpepper <ry...@ccs.neu.edu> wrote:
> On 03/13/2017 12:49 PM, David Storrs wrote:
>> Assuming I've understood all that correctly, my last question would be
>> how to get around the 'can't do prepare with a virtual connection' issue
>> for situations where I've been passed a connection (perhaps from third
>> party code) and it might or might not be virtual. First, to dispose of
>> some quibbles:
>> - One answer is "well, don't do that." Write a contract on the function
>> that mandates the connection being non-virtual.
>> - Another is "well, don't do that." Test if the connection is virtual
>> and, if so, don't use prepare.
>> - Another is "well, don't do that." Pass around a dsn instead of a VC
>> and generate connections as needed.
>> None of these is terribly satisfying. The first violates the principle
>> of "be generous in what you accept and strict in what you emit", the
>> second gives up a lot of speed if we were in a situation where we wanted
>> to use 'prepare' in the first place, and the third isn't feasible since
>> I won't always have control over what a third-party library emits.
>> My ideal solution would be something like this:
>> (define (myfunc a-handle)
>> (define dbh
>> (if (virtual-connection? a-handle)
>> (my-function-to-do-connect-with-dsn (get-dsn-from a-handle))
>> (define sth (prepare dbh "select foo from bar where baz = $1"))
>> In other words, check if the connection is virtual and, if so, extract
>> the dsn from it and use that to create a non-virtual connection.
>> Is there a way to do that? I've been through the db module docs and
>> Google but not found a way. Did I miss something?
> There's no way to do that at the moment.
> If you are using `prepare` just for speed, it might help to know that most
> base connections have an internal statement cache that maps SQL strings to
> prepared statement objects. The cache is only used inside of transactions,
> though, to avoid issues with concurrent schema changes.
> You can check on statement caching by setting the #:debug? argument when
> connecting. Aside from lots of other noise, queries will print out whether
> they are using the statement cache. Here's an example:
> > (define c (dsn-connect 'pg #:debug? #t))
> > (start-transaction c)
> ** in managed transaction
> > (query c "select 1")
> ** caching statement
> > (query c "select 1")
> ** using cached statement
> Could that replace your use of `prepare`?
<goes and experiments>
Looks like inside a transaction all statements are prepared. Cool. If so,
that's very nearly perfect -- it adds and removes all the concerns that
transactions always add/remove, but offhand I can't think of a case where
doing it in a transaction would be a problem.
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