Re: [HACKERS] Frontend/backend protocol improvements proposal (request).

I'm moving this discussion to -general.

Dmitriy Igrishin wrote:
>>> While developing a C++ client library for Postgres I felt lack of extra
>>> information in command tags in the CommandComplete (B) message [...]
>>> for the following commands:

>> It seems like bad design to me to keep a list of prepared statements
>> on the client side when it is already kept on the server side
>> (accessible with the pg_prepared_statements view).
>> 
>> What's wrong with the following:
>> If the user wants to deallocate an individual prepared statement,
>> just send "DEALLOCATE <statement name>" to the server.  If the
>> statement does not exist, the server will return an error.
>> If the user wants to deallocate all statements, just send
>> "DEALLOCATE ALL".
>> Why do you need to track prepared statements on the client side?
> 
> 
> Nothing wrong if the user wants to deal with scary and cumbersome code.
> As library author, I want to help people make things simpler.

I don't think that anything would change on the user end.

> To understand me, please look at the pseudo C++ code below.
> 
> 
> // A class designed to work with prepared statements
> class Prepared_statement {
> 
> public:
>   // Methods to generate a Bind message, like
>   Prepared_statement* bind(Position, Value);
>   // ... and more
>   // Methods to send Execute message, like
>   void execute();
>   void execute_async();
> };
> 
> class Connection {
> public:
>   // many stuff ...
>   void close();
> 
>   Prepared_statement* prepare(Name, Query);
>   void prepare_async(Statement);
> 
>   // Make yet another instance of prepared statement.
>   Prepared_statement* prepared_statement(Name);
> 
>   // etc.
> };
> 
> The Connection class is a factory for Prepared_statement instances.
> As you can see, the Connection::prepare() returns new instance of
> *synchronously* prepared statement. Next, the user can bind values
> and execute the statement, like this:
> 
> void f(Connection* cn)
> {
>   // Prepare unnamed statement and execute it.
>   cn->prepare("SELECT $1::text")->bind(0, "Albe")->execute();
>   // Ps: don't worry about absence of delete; We are using smart pointers :-)
> }
> 
> But there is a another possible case:
> 
> void f(Connection* cn)
> {
>   Prepared_statement* ps = cn->prepare("SELECT $1::text");
>   cn->close(); // THIS SHOULD invalidate all Prepared_statement instances ...
>   ps->bind(0, "Albe"); // ... to throw the exception here
> }

Attempting to send a bind message over a closed connection
will result in a PostgreSQL error.  All you have to do is wrap
that into an exception of your liking.

> Moreover, consider:
> 
> void f(Connection* cn)
> {
>   Prepared_statement* ps1 = cn->prepare("ps1", "SELECT $1::text");
> 
>   cn->deallocate("ps1"); // THIS invalidates ps1 object...

Shouldn't that be
  cn->deallocate(ps1);
without quotes?

>   ps1->bind(0, "Albe"); // ... to throw the exception here
> 
> 
>   Prepared_statement* ps2 = cn->prepare("ps2", "SELECT $1::text");
> 
>   cn->perform("DEALLOCATE ps2"); // THIS SHOULD ALSO invalidate ps2 object...
>   ps2->bind(0, "Albe"); // ... to throw the exception here
> 
> }

Again, sending a bind message for a deallocated prepared statement
will cause a PostgreSQL error automatically.

> In the latter case when the user deallocates named prepared statement directly,
> the implementation of Connection can invalidates the prepared statement (ps2) by
> analyzing and parsing CommandComplete command tag to get it's name.
> 
> And please note, that the user can send DEALLOCATE asynchronously. And there is
> only two ways to get the prepared statement (or another session object's) name:
>   1) Parse the SQL command which the user is attempts to send;
>   2) Just get it from CommandComplete command tag.
> 
> I beleive that the 1) is a 100% bad idea.
> 
> PS: this C++11 library is not publicaly available yet, but I hope it will this year.

I still think that it is a bad idea to track this on the client side.

What's wrong with throwing an exception when you get a PostgreSQL error?
If you want to distinguish between certain error conditions,
you can use the SQLSTATE.  For example, trying to execute a deallocated
statement would cause SQLSTATE 26000.

Yours,
Laurenz Albe