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postgres/src/include/executor/nodeIncrementalSort.h

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Implement Incremental Sort Incremental Sort is an optimized variant of multikey sort for cases when the input is already sorted by a prefix of the requested sort keys. For example when the relation is already sorted by (key1, key2) and we need to sort it by (key1, key2, key3) we can simply split the input rows into groups having equal values in (key1, key2), and only sort/compare the remaining column key3. This has a number of benefits: - Reduced memory consumption, because only a single group (determined by values in the sorted prefix) needs to be kept in memory. This may also eliminate the need to spill to disk. - Lower startup cost, because Incremental Sort produce results after each prefix group, which is beneficial for plans where startup cost matters (like for example queries with LIMIT clause). We consider both Sort and Incremental Sort, and decide based on costing. The implemented algorithm operates in two different modes: - Fetching a minimum number of tuples without check of equality on the prefix keys, and sorting on all columns when safe. - Fetching all tuples for a single prefix group and then sorting by comparing only the remaining (non-prefix) keys. We always start in the first mode, and employ a heuristic to switch into the second mode if we believe it's beneficial - the goal is to minimize the number of unnecessary comparions while keeping memory consumption below work_mem. This is a very old patch series. The idea was originally proposed by Alexander Korotkov back in 2013, and then revived in 2017. In 2018 the patch was taken over by James Coleman, who wrote and rewrote most of the current code. There were many reviewers/contributors since 2013 - I've done my best to pick the most active ones, and listed them in this commit message. Author: James Coleman, Alexander Korotkov Reviewed-by: Tomas Vondra, Andreas Karlsson, Marti Raudsepp, Peter Geoghegan, Robert Haas, Thomas Munro, Antonin Houska, Andres Freund, Alexander Kuzmenkov Discussion: https://postgr.es/m/CAPpHfdscOX5an71nHd8WSUH6GNOCf=V7wgDaTXdDd9=goN-gfA@mail.gmail.com Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com
6 years ago
/*-------------------------------------------------------------------------
*
* nodeIncrementalSort.h
*
* Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/executor/nodeIncrementalSort.h
*
*-------------------------------------------------------------------------
*/
#ifndef NODEINCREMENTALSORT_H
#define NODEINCREMENTALSORT_H
#include "access/parallel.h"
#include "nodes/execnodes.h"
extern IncrementalSortState *ExecInitIncrementalSort(IncrementalSort *node, EState *estate, int eflags);
extern void ExecEndIncrementalSort(IncrementalSortState *node);
extern void ExecReScanIncrementalSort(IncrementalSortState *node);
/* parallel instrumentation support */
extern void ExecIncrementalSortEstimate(IncrementalSortState *node, ParallelContext *pcxt);
extern void ExecIncrementalSortInitializeDSM(IncrementalSortState *node, ParallelContext *pcxt);
extern void ExecIncrementalSortInitializeWorker(IncrementalSortState *node, ParallelWorkerContext *pcxt);
extern void ExecIncrementalSortRetrieveInstrumentation(IncrementalSortState *node);
#endif /* NODEINCREMENTALSORT_H */