libpqxx  7.3.0
pqxx::pipeline Class Reference

Processes several queries in FIFO manner, optimized for high throughput. More...

#include <pipeline.hxx>

Inheritance diagram for pqxx::pipeline:

Public Types

using query_id = long

Public Member Functions

 pipeline (pipeline const &)=delete
pipelineoperator= (pipeline const &)=delete
 pipeline (transaction_base &t)
 pipeline (transaction_base &t, std::string_view tname)
 ~pipeline () noexcept
query_id insert (std::string_view)
 Add query to the pipeline. More...
void complete ()
 Wait for all ongoing or pending operations to complete, and detach. More...
void flush ()
 Forget all ongoing or pending operations and retrieved results. More...
void cancel ()
 Cancel ongoing query, if any. More...
bool is_finished (query_id) const
 Is result for given query available? More...
result retrieve (query_id qid)
 Retrieve result for given query. More...
std::pair< query_id, resultretrieve ()
 Retrieve oldest unretrieved result (possibly wait for one). More...
bool empty () const noexcept
int retain (int retain_max=2)
void resume ()
 Resume retained query emission. Harmless when not needed. More...

Detailed Description

Processes several queries in FIFO manner, optimized for high throughput.

Use a pipeline if you want to keep doing useful work while your queries are executing. Result retrieval is decoupled from execution request; queries "go in at the front" and results "come out the back."

Actually, you can retrieve the results in any order if you want, but it may lead to surprising "time travel" effects if any of the queries fails. In particular, syntax errors in the queries can confuse things and show up too early in the stream of results.

Generally, if any of the queries fails, it will throw an exception at the point where you request its result. But it may happen earlier, especially if you request results out of chronological order.

Member Typedef Documentation

◆ query_id

Constructor & Destructor Documentation

◆ pipeline() [1/3]

pqxx::pipeline::pipeline ( pipeline const &  )

◆ pipeline() [2/3]

pqxx::pipeline::pipeline ( transaction_base t)

◆ pipeline() [3/3]

pqxx::pipeline::pipeline ( transaction_base t,
std::string_view  tname 

◆ ~pipeline()

pqxx::pipeline::~pipeline ( )

References cancel().

Member Function Documentation

◆ cancel()

void pqxx::pipeline::cancel ( )

Cancel ongoing query, if any.

May cancel any or all of the queries that have been inserted at this point whose results have not yet been retrieved. If the pipeline lives in a backend transaction, that transaction may be left in a nonfunctional state in which it can only be aborted.

Therefore, either use this function in a nontransaction, or abort the transaction after calling it.

Referenced by ~pipeline().

◆ complete()

void pqxx::pipeline::complete ( )

Wait for all ongoing or pending operations to complete, and detach.

Detaches from the transaction when done.

This does not produce the queries' results, so it may not report any errors which may have occurred in their execution. To be sure that your statements succeeded, call retrieve() until the pipeline is empty.

◆ empty()

bool pqxx::pipeline::empty ( ) const

◆ flush()

void pqxx::pipeline::flush ( )

Forget all ongoing or pending operations and retrieved results.

Queries already sent to the backend may still be completed, depending on implementation and timing.

Any error state (unless caused by an internal error) will also be cleared. This is mostly useful in a nontransaction, since a backend transaction is aborted automatically when an error occurs.

Detaches from the transaction when done.

◆ insert()

pqxx::pipeline::query_id pqxx::pipeline::insert ( std::string_view  q)

Add query to the pipeline.

Queries accumulate in the pipeline, which sends them to the backend in a batch separated by semicolons. The queries you insert must not use this trick themselves, or the pipeline will get hopelessly confused!

Identifier for this query, unique only within this pipeline.

◆ is_finished()

bool pqxx::pipeline::is_finished ( pipeline::query_id  q) const

Is result for given query available?

◆ operator=()

pipeline& pqxx::pipeline::operator= ( pipeline const &  )

◆ resume()

void pqxx::pipeline::resume ( )

Resume retained query emission. Harmless when not needed.

References pqxx::check_cast(), retrieve(), and pqxx::separated_list().

Referenced by retain().

◆ retain()

int pqxx::pipeline::retain ( int  retain_max = 2)

Set maximum number of queries to retain before issuing them to the backend. The pipeline will perform better if multiple queries are issued at once, but retaining queries until the results are needed (as opposed to issuing them to the backend immediately) may negate any performance benefits the pipeline can offer.

Recommended practice is to set this value no higher than the number of queries you intend to insert at a time.

retain_maxA nonnegative "retention capacity;" passing zero will cause queries to be issued immediately
Old retention capacity

References resume().

◆ retrieve() [1/2]

result pqxx::pipeline::retrieve ( query_id  qid)

Retrieve result for given query.

If the query failed for whatever reason, this will throw an exception. The function will block if the query has not finished yet.

If results are retrieved out-of-order, i.e. in a different order than the one in which their queries were inserted, errors may "propagate" to subsequent queries.

◆ retrieve() [2/2]

std::pair< pqxx::pipeline::query_id, pqxx::result > pqxx::pipeline::retrieve ( )

Retrieve oldest unretrieved result (possibly wait for one).

The query's identifier and its result set.

Referenced by resume().

The documentation for this class was generated from the following files: