stream_query with parameters

[sulakm]

Hello,

Is it possible to simply replace "query" with "stream" in this construct ?

for (auto[...] : tx.query<...>("select ... =$1", params{key}) { ... }

=> for (auto[...] : tx.stream<...>("select ... =$1", params{key}) { ... }

Select with tx.query works fine.
With tx.stream code compiles but I get this exception:
ERROR: bind message supplies 1 parameters, but prepared statement "" requires 0

I took a peek at your source code and both methods internally calls exec(sql,params) with one difference: stream wraps sql query in copy to stdout.
So I tried tx.query("copy (select ... =$1) to stdout", params{key) and this time I got the same exception as with tx.stream.
I quess prepared statements with "copy" are not supported either in libpq or the server.

[jtv]

The error message suggests to me that the implementation of parameterised statements in libpq uses a prepared statement. (The empty name has a special meaning — basically that's the single-use prepared statement.)

Streaming a parameterised statement ought to work, from libpqxx's perspective. But apparently the frontend/backend protocol prevents it! I guess I never thought to write a test for this because it's so simple and regular, API-wise, that I didn't think of it as a separate test case.

[jtv]

It's a bit painful — you'll have to insert the parameters into the query string yourself. Just always remember to quote and escape any string values using the connection's quote() function.