32.5. Pipeline Mode

After entering pipeline mode, the application dispatches requests using 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 , PQsendDescribePortal , PQsendClosePrepared , and PQsendClosePortal 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 either PQpipelineSync or PQsendPipelineSync , 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 32.5.1.4 .

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 or PQsendPipelineSync 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 32.5.1.3 .

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 or PQsendPipelineSync 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 .

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.