libpq pipeline mode allows applications to send a query without having to read the result of the previously sent query. Taking advantage of the pipeline mode, a client will wait less for the server, since multiple queries/results can be sent/received in a single network transaction.
While pipeline mode provides a significant performance boost, writing clients using the pipeline mode is more complex because it involves managing a queue of pending queries and finding which result corresponds to which query in the queue.
Pipeline mode also generally consumes more memory on both the client and server, though careful and aggressive management of the send/receive queue can mitigate this. This applies whether or not the connection is in blocking or non-blocking mode.
While the pipeline API was introduced in PostgreSQL 14, it is a client-side feature which doesn't require special server support and works on any server that supports the v3 extended query protocol.
To issue pipelines, the application must switch the connection
into pipeline mode,
which is done with PQenterPipelineMode.
PQpipelineStatus can be used
to test whether pipeline mode is active.
In pipeline mode, only asynchronous operations
are permitted, command strings containing multiple SQL commands are
disallowed, and so is COPY.
Using synchronous command execution functions
such as PQfn,
PQexec,
PQexecParams,
PQprepare,
PQexecPrepared,
PQdescribePrepared,
PQdescribePortal,
is an error condition.
Once all dispatched commands have had their results processed, and
the end pipeline result has been consumed, the application may return
to non-pipelined mode with PQexitPipelineMode.
It is best to use pipeline mode with libpq in non-blocking mode. If used in blocking mode it is possible for a client/server deadlock to occur. [15]
After entering pipeline mode, the application dispatches requests using
PQsendQuery,
PQsendQueryParams,
or its prepared-query sibling
PQsendQueryPrepared.
These requests are queued on the client-side until flushed to the server;
this occurs when PQpipelineSync is used to
establish a synchronization point in the pipeline,
or when PQflush is called.
The functions PQsendPrepare,
PQsendDescribePrepared, and
PQsendDescribePortal also work in pipeline mode.
Result processing is described below.
The server executes statements, and returns results, in the order the
client sends them. The server will begin executing the commands in the
pipeline immediately, not waiting for the end of the pipeline.
Note that results are buffered on the server side; the server flushes
that buffer when a synchronization point is established with
PQpipelineSync, or when
PQsendFlushRequest is called.
If any statement encounters an error, the server aborts the current
transaction and does not execute any subsequent command in the queue
until the next synchronization point;
a PGRES_PIPELINE_ABORTED result is produced for
each such command.
(This remains true even if the commands in the pipeline would rollback
the transaction.)
Query processing resumes after the synchronization point.
It's fine for one operation to depend on the results of a prior one; for example, one query may define a table that the next query in the same pipeline uses. Similarly, an application may create a named prepared statement and execute it with later statements in the same pipeline.
To process the result of one query in a pipeline, the application calls
PQgetResult repeatedly and handles each result
until PQgetResult returns null.
The result from the next query in the pipeline may then be retrieved using
PQgetResult again and the cycle repeated.
The application handles individual statement results as normal.
When the results of all the queries in the pipeline have been
returned, PQgetResult returns a result
containing the status value PGRES_PIPELINE_SYNC
The client may choose to defer result processing until the complete pipeline has been sent, or interleave that with sending further queries in the pipeline; see Section 34.5.1.4.
To enter single-row mode, call PQsetSingleRowMode
before retrieving results with PQgetResult.
This mode selection is effective only for the query currently
being processed. For more information on the use of
PQsetSingleRowMode,
refer to Section 34.6.
PQgetResult behaves the same as for normal
asynchronous processing except that it may contain the new
PGresult types PGRES_PIPELINE_SYNC
and PGRES_PIPELINE_ABORTED.
PGRES_PIPELINE_SYNC is reported exactly once for each
PQpipelineSync at the corresponding point
in the pipeline.
PGRES_PIPELINE_ABORTED is emitted in place of a normal
query result for the first error and all subsequent results
until the next PGRES_PIPELINE_SYNC;
see Section 34.5.1.3.
PQisBusy, PQconsumeInput, etc
operate as normal when processing pipeline results. In particular,
a call to PQisBusy in the middle of a pipeline
returns 0 if the results for all the queries issued so far have been
consumed.
libpq does not provide any information to the
application about the query currently being processed (except that
PQgetResult returns null to indicate that we start
returning the results of next query). The application must keep track
of the order in which it sent queries, to associate them with their
corresponding results.
Applications will typically use a state machine or a FIFO queue for this.
From the client's perspective, after PQresultStatus
returns PGRES_FATAL_ERROR,
the pipeline is flagged as aborted.
PQresultStatus will report a
PGRES_PIPELINE_ABORTED result for each remaining queued
operation in an aborted pipeline. The result for
PQpipelineSync is reported as
PGRES_PIPELINE_SYNC to signal the end of the aborted pipeline
and resumption of normal result processing.
The client must process results with
PQgetResult during error recovery.
If the pipeline used an implicit transaction, then operations that have
already executed are rolled back and operations that were queued to follow
the failed operation are skipped entirely. The same behavior holds if the
pipeline starts and commits a single explicit transaction (i.e. the first
statement is BEGIN and the last is
COMMIT) except that the session remains in an aborted
transaction state at the end of the pipeline. If a pipeline contains
multiple explicit transactions, all transactions that
committed prior to the error remain committed, the currently in-progress
transaction is aborted, and all subsequent operations are skipped completely,
including subsequent transactions. If a pipeline synchronization point
occurs with an explicit transaction block in aborted state, the next pipeline
will become aborted immediately unless the next command puts the transaction
in normal mode with ROLLBACK.
The client must not assume that work is committed when it
sends a COMMIT — only when the
corresponding result is received to confirm the commit is complete.
Because errors arrive asynchronously, the application needs to be able to
restart from the last received committed change and
resend work done after that point if something goes wrong.
To avoid deadlocks on large pipelines the client should be structured
around a non-blocking event loop using operating system facilities
such as select, poll,
WaitForMultipleObjectEx, etc.
The client application should generally maintain a queue of work remaining to be dispatched and a queue of work that has been dispatched but not yet had its results processed. When the socket is writable it should dispatch more work. When the socket is readable it should read results and process them, matching them up to the next entry in its corresponding results queue. Based on available memory, results from the socket should be read frequently: there's no need to wait until the pipeline end to read the results. Pipelines should be scoped to logical units of work, usually (but not necessarily) one transaction per pipeline. There's no need to exit pipeline mode and re-enter it between pipelines, or to wait for one pipeline to finish before sending the next.
An example using select() and a simple state
machine to track sent and received work is in
src/test/modules/libpq_pipeline/libpq_pipeline.c
in the PostgreSQL source distribution.
PQpipelineStatusReturns the current pipeline mode status of the libpq connection.
PGpipelineStatus PQpipelineStatus(const PGconn *conn);
PQpipelineStatus can return one of the following values:
PQ_PIPELINE_ON
The libpq connection is in pipeline mode.
PQ_PIPELINE_OFF
The libpq connection is not in pipeline mode.
PQ_PIPELINE_ABORTED
The libpq connection is in pipeline
mode and an error occurred while processing the current pipeline.
The aborted flag is cleared when PQgetResult
returns a result of type PGRES_PIPELINE_SYNC.
PQenterPipelineModeCauses a connection to enter pipeline mode if it is currently idle or already in pipeline mode.
int PQenterPipelineMode(PGconn *conn);
Returns 1 for success. Returns 0 and has no effect if the connection is not currently idle, i.e., it has a result ready, or it is waiting for more input from the server, etc. This function does not actually send anything to the server, it just changes the libpq connection state.
PQexitPipelineModeCauses a connection to exit pipeline mode if it is currently in pipeline mode with an empty queue and no pending results.
int PQexitPipelineMode(PGconn *conn);
Returns 1 for success. Returns 1 and takes no action if not in
pipeline mode. If the current statement isn't finished processing,
or PQgetResult has not been called to collect
results from all previously sent query, returns 0 (in which case,
use PQerrorMessage to get more information
about the failure).
PQpipelineSyncMarks a synchronization point in a pipeline by sending a sync message and flushing the send buffer. This serves as the delimiter of an implicit transaction and an error recovery point; see Section 34.5.1.3.
int PQpipelineSync(PGconn *conn);
Returns 1 for success. Returns 0 if the connection is not in pipeline mode or sending a sync message failed.
PQsendFlushRequestSends a request for the server to flush its output buffer.
int PQsendFlushRequest(PGconn *conn);
Returns 1 for success. Returns 0 on any failure.
The server flushes its output buffer automatically as a result of
PQpipelineSync being called, or
on any request when not in pipeline mode; this function is useful
to cause the server to flush its output buffer in pipeline mode
without establishing a synchronization point.
Note that the request is not itself flushed to the server automatically;
use PQflush if necessary.
Much like asynchronous query mode, there is no meaningful performance overhead when using pipeline mode. It increases client application complexity, and extra caution is required to prevent client/server deadlocks, but pipeline mode can offer considerable performance improvements, in exchange for increased memory usage from leaving state around longer.
Pipeline mode is most useful when the server is distant, i.e., network latency (“ping time”) is high, and also when many small operations are being performed in rapid succession. There is usually less benefit in using pipelined commands when each query takes many multiples of the client/server round-trip time to execute. A 100-statement operation run on a server 300 ms round-trip-time away would take 30 seconds in network latency alone without pipelining; with pipelining it may spend as little as 0.3 s waiting for results from the server.
Use pipelined commands when your application does lots of small
INSERT, UPDATE and
DELETE operations that can't easily be transformed
into operations on sets, or into a COPY operation.
Pipeline mode is not useful when information from one operation is required by the client to produce the next operation. In such cases, the client would have to introduce a synchronization point and wait for a full client/server round-trip to get the results it needs. However, it's often possible to adjust the client design to exchange the required information server-side. Read-modify-write cycles are especially good candidates; for example:
BEGIN; SELECT x FROM mytable WHERE id = 42 FOR UPDATE; -- result: x=2 -- client adds 1 to x: UPDATE mytable SET x = 3 WHERE id = 42; COMMIT;
could be much more efficiently done with:
UPDATE mytable SET x = x + 1 WHERE id = 42;
Pipelining is less useful, and more complex, when a single pipeline contains multiple transactions (see Section 34.5.1.3).
[15] The client will block trying to send queries to the server, but the server will block trying to send results to the client from queries it has already processed. This only occurs when the client sends enough queries to fill both its output buffer and the server's receive buffer before it switches to processing input from the server, but it's hard to predict exactly when that will happen.