847 lines
29 KiB
C
847 lines
29 KiB
C
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/*-------------------------------------------------------------------------
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*
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* nbtsplitloc.c
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* Choose split point code for Postgres btree implementation.
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*
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* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* src/backend/access/nbtree/nbtsplitloc.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/nbtree.h"
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#include "storage/lmgr.h"
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/* limits on split interval (default strategy only) */
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#define MAX_LEAF_INTERVAL 9
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#define MAX_INTERNAL_INTERVAL 18
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typedef enum
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{
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/* strategy for searching through materialized list of split points */
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SPLIT_DEFAULT, /* give some weight to truncation */
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SPLIT_MANY_DUPLICATES, /* find minimally distinguishing point */
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SPLIT_SINGLE_VALUE /* leave left page almost full */
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} FindSplitStrat;
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typedef struct
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{
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/* details of free space left by split */
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int16 curdelta; /* current leftfree/rightfree delta */
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int16 leftfree; /* space left on left page post-split */
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int16 rightfree; /* space left on right page post-split */
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/* split point identifying fields (returned by _bt_findsplitloc) */
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OffsetNumber firstoldonright; /* first item on new right page */
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bool newitemonleft; /* new item goes on left, or right? */
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} SplitPoint;
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typedef struct
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{
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/* context data for _bt_recsplitloc */
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Relation rel; /* index relation */
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Page page; /* page undergoing split */
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IndexTuple newitem; /* new item (cause of page split) */
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Size newitemsz; /* size of newitem (includes line pointer) */
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bool is_leaf; /* T if splitting a leaf page */
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bool is_rightmost; /* T if splitting rightmost page on level */
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OffsetNumber newitemoff; /* where the new item is to be inserted */
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int leftspace; /* space available for items on left page */
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int rightspace; /* space available for items on right page */
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int olddataitemstotal; /* space taken by old items */
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Size minfirstrightsz; /* smallest firstoldonright tuple size */
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/* candidate split point data */
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int maxsplits; /* maximum number of splits */
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int nsplits; /* current number of splits */
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SplitPoint *splits; /* all candidate split points for page */
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int interval; /* current range of acceptable split points */
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} FindSplitData;
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static void _bt_recsplitloc(FindSplitData *state,
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OffsetNumber firstoldonright, bool newitemonleft,
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int olddataitemstoleft, Size firstoldonrightsz);
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static void _bt_deltasortsplits(FindSplitData *state, double fillfactormult,
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bool usemult);
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static int _bt_splitcmp(const void *arg1, const void *arg2);
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static OffsetNumber _bt_bestsplitloc(FindSplitData *state, int perfectpenalty,
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bool *newitemonleft);
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static int _bt_strategy(FindSplitData *state, SplitPoint *leftpage,
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SplitPoint *rightpage, FindSplitStrat *strategy);
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static void _bt_interval_edges(FindSplitData *state,
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SplitPoint **leftinterval, SplitPoint **rightinterval);
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static inline int _bt_split_penalty(FindSplitData *state, SplitPoint *split);
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static inline IndexTuple _bt_split_lastleft(FindSplitData *state,
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SplitPoint *split);
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static inline IndexTuple _bt_split_firstright(FindSplitData *state,
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SplitPoint *split);
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/*
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* _bt_findsplitloc() -- find an appropriate place to split a page.
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*
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* The main goal here is to equalize the free space that will be on each
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* split page, *after accounting for the inserted tuple*. (If we fail to
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* account for it, we might find ourselves with too little room on the page
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* that it needs to go into!)
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*
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* If the page is the rightmost page on its level, we instead try to arrange
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* to leave the left split page fillfactor% full. In this way, when we are
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* inserting successively increasing keys (consider sequences, timestamps,
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* etc) we will end up with a tree whose pages are about fillfactor% full,
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* instead of the 50% full result that we'd get without this special case.
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* This is the same as nbtsort.c produces for a newly-created tree. Note
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* that leaf and nonleaf pages use different fillfactors. Note also that
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* there are a number of further special cases where fillfactor is not
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* applied in the standard way.
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*
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* We are passed the intended insert position of the new tuple, expressed as
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* the offsetnumber of the tuple it must go in front of (this could be
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* maxoff+1 if the tuple is to go at the end). The new tuple itself is also
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* passed, since it's needed to give some weight to how effective suffix
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* truncation will be. The implementation picks the split point that
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* maximizes the effectiveness of suffix truncation from a small list of
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* alternative candidate split points that leave each side of the split with
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* about the same share of free space. Suffix truncation is secondary to
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* equalizing free space, except in cases with large numbers of duplicates.
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* Note that it is always assumed that caller goes on to perform truncation,
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* even with pg_upgrade'd indexes where that isn't actually the case
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* (!heapkeyspace indexes). See nbtree/README for more information about
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* suffix truncation.
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*
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* We return the index of the first existing tuple that should go on the
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* righthand page, plus a boolean indicating whether the new tuple goes on
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* the left or right page. The bool is necessary to disambiguate the case
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* where firstright == newitemoff.
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*/
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OffsetNumber
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_bt_findsplitloc(Relation rel,
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Page page,
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OffsetNumber newitemoff,
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Size newitemsz,
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IndexTuple newitem,
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bool *newitemonleft)
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{
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BTPageOpaque opaque;
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int leftspace,
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rightspace,
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olddataitemstotal,
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olddataitemstoleft,
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perfectpenalty,
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leaffillfactor;
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FindSplitData state;
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FindSplitStrat strategy;
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ItemId itemid;
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OffsetNumber offnum,
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maxoff,
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foundfirstright;
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double fillfactormult;
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bool usemult;
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SplitPoint leftpage,
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rightpage;
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opaque = (BTPageOpaque) PageGetSpecialPointer(page);
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maxoff = PageGetMaxOffsetNumber(page);
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/* Total free space available on a btree page, after fixed overhead */
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leftspace = rightspace =
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PageGetPageSize(page) - SizeOfPageHeaderData -
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MAXALIGN(sizeof(BTPageOpaqueData));
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/* The right page will have the same high key as the old page */
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if (!P_RIGHTMOST(opaque))
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{
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itemid = PageGetItemId(page, P_HIKEY);
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rightspace -= (int) (MAXALIGN(ItemIdGetLength(itemid)) +
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sizeof(ItemIdData));
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}
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/* Count up total space in data items before actually scanning 'em */
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olddataitemstotal = rightspace - (int) PageGetExactFreeSpace(page);
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leaffillfactor = RelationGetFillFactor(rel, BTREE_DEFAULT_FILLFACTOR);
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/* Passed-in newitemsz is MAXALIGNED but does not include line pointer */
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newitemsz += sizeof(ItemIdData);
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state.rel = rel;
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state.page = page;
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state.newitem = newitem;
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state.newitemsz = newitemsz;
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state.is_leaf = P_ISLEAF(opaque);
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state.is_rightmost = P_RIGHTMOST(opaque);
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state.leftspace = leftspace;
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state.rightspace = rightspace;
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state.olddataitemstotal = olddataitemstotal;
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state.minfirstrightsz = SIZE_MAX;
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state.newitemoff = newitemoff;
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/*
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* maxsplits should never exceed maxoff because there will be at most as
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* many candidate split points as there are points _between_ tuples, once
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* you imagine that the new item is already on the original page (the
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* final number of splits may be slightly lower because not all points
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* between tuples will be legal).
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*/
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state.maxsplits = maxoff;
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state.splits = palloc(sizeof(SplitPoint) * state.maxsplits);
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state.nsplits = 0;
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/*
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* Scan through the data items and calculate space usage for a split at
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* each possible position. We start at the first data offset rather than
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* the second data offset to handle the "newitemoff == first data offset"
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* case (any other split whose firstoldonright is the first data offset
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* can't be legal, though, and so won't actually end up being recorded in
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* first loop iteration).
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*/
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olddataitemstoleft = 0;
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for (offnum = P_FIRSTDATAKEY(opaque);
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offnum <= maxoff;
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offnum = OffsetNumberNext(offnum))
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{
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Size itemsz;
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itemid = PageGetItemId(page, offnum);
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itemsz = MAXALIGN(ItemIdGetLength(itemid)) + sizeof(ItemIdData);
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/*
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* Will the new item go to left or right of split?
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*/
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if (offnum > newitemoff)
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_bt_recsplitloc(&state, offnum, true, olddataitemstoleft, itemsz);
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else if (offnum < newitemoff)
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_bt_recsplitloc(&state, offnum, false, olddataitemstoleft, itemsz);
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else
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{
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/* may need to record a split on one or both sides of new item */
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_bt_recsplitloc(&state, offnum, true, olddataitemstoleft, itemsz);
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_bt_recsplitloc(&state, offnum, false, olddataitemstoleft, itemsz);
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}
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olddataitemstoleft += itemsz;
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}
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/*
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* If the new item goes as the last item, record the split point that
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* leaves all the old items on the left page, and the new item on the
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* right page. This is required because a split that leaves the new item
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* as the firstoldonright won't have been reached within the loop.
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*/
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Assert(olddataitemstoleft == olddataitemstotal);
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if (newitemoff > maxoff)
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_bt_recsplitloc(&state, newitemoff, false, olddataitemstotal, 0);
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/*
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* I believe it is not possible to fail to find a feasible split, but just
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* in case ...
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*/
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if (state.nsplits == 0)
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elog(ERROR, "could not find a feasible split point for index \"%s\"",
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RelationGetRelationName(rel));
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/*
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* Start search for a split point among list of legal split points. Give
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* primary consideration to equalizing available free space in each half
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* of the split initially (start with default strategy), while applying
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* rightmost optimization where appropriate. Either of the two other
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* fallback strategies may be required for cases with a large number of
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* duplicates around the original/space-optimal split point.
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*
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* Default strategy gives some weight to suffix truncation in deciding a
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* split point on leaf pages. It attempts to select a split point where a
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* distinguishing attribute appears earlier in the new high key for the
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* left side of the split, in order to maximize the number of trailing
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* attributes that can be truncated away. Only candidate split points
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* that imply an acceptable balance of free space on each side are
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* considered.
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*/
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if (!state.is_leaf)
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{
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/* fillfactormult only used on rightmost page */
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usemult = state.is_rightmost;
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fillfactormult = BTREE_NONLEAF_FILLFACTOR / 100.0;
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}
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else if (state.is_rightmost)
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{
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/* Rightmost leaf page -- fillfactormult always used */
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usemult = true;
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fillfactormult = leaffillfactor / 100.0;
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}
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else
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{
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/* Other leaf page. 50:50 page split. */
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usemult = false;
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/* fillfactormult not used, but be tidy */
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fillfactormult = 0.50;
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}
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/*
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* Set an initial limit on the split interval/number of candidate split
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* points as appropriate. The "Prefix B-Trees" paper refers to this as
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* sigma l for leaf splits and sigma b for internal ("branch") splits.
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* It's hard to provide a theoretical justification for the initial size
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* of the split interval, though it's clear that a small split interval
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* makes suffix truncation much more effective without noticeably
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* affecting space utilization over time.
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*/
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state.interval = Min(Max(1, state.nsplits * 0.05),
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state.is_leaf ? MAX_LEAF_INTERVAL :
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MAX_INTERNAL_INTERVAL);
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/*
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* Save leftmost and rightmost splits for page before original ordinal
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* sort order is lost by delta/fillfactormult sort
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*/
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leftpage = state.splits[0];
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rightpage = state.splits[state.nsplits - 1];
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/* Give split points a fillfactormult-wise delta, and sort on deltas */
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_bt_deltasortsplits(&state, fillfactormult, usemult);
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/*
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* Determine if default strategy/split interval will produce a
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* sufficiently distinguishing split, or if we should change strategies.
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* Alternative strategies change the range of split points that are
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* considered acceptable (split interval), and possibly change
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* fillfactormult, in order to deal with pages with a large number of
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* duplicates gracefully.
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*
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* Pass low and high splits for the entire page (including even newitem).
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* These are used when the initial split interval encloses split points
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* that are full of duplicates, and we need to consider if it's even
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* possible to avoid appending a heap TID.
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*/
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perfectpenalty = _bt_strategy(&state, &leftpage, &rightpage, &strategy);
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if (strategy == SPLIT_DEFAULT)
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{
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/*
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* Default strategy worked out (always works out with internal page).
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* Original split interval still stands.
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*/
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}
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/*
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* Many duplicates strategy is used when a heap TID would otherwise be
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* appended, but the page isn't completely full of logical duplicates.
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*
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* The split interval is widened to include all legal candidate split
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* points. There may be a few as two distinct values in the whole-page
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* split interval. Many duplicates strategy has no hard requirements for
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* space utilization, though it still keeps the use of space balanced as a
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* non-binding secondary goal (perfect penalty is set so that the
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* first/lowest delta split points that avoids appending a heap TID is
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* used).
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*
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* Single value strategy is used when it is impossible to avoid appending
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* a heap TID. It arranges to leave the left page very full. This
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* maximizes space utilization in cases where tuples with the same
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* attribute values span many pages. Newly inserted duplicates will tend
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* to have higher heap TID values, so we'll end up splitting to the right
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* consistently. (Single value strategy is harmless though not
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* particularly useful with !heapkeyspace indexes.)
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*/
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else if (strategy == SPLIT_MANY_DUPLICATES)
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{
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Assert(state.is_leaf);
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/* No need to resort splits -- no change in fillfactormult/deltas */
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state.interval = state.nsplits;
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}
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else if (strategy == SPLIT_SINGLE_VALUE)
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{
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Assert(state.is_leaf);
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/* Split near the end of the page */
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usemult = true;
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fillfactormult = BTREE_SINGLEVAL_FILLFACTOR / 100.0;
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/* Resort split points with new delta */
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_bt_deltasortsplits(&state, fillfactormult, usemult);
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/* Appending a heap TID is unavoidable, so interval of 1 is fine */
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state.interval = 1;
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}
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/*
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* Search among acceptable split points (using final split interval) for
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* the entry that has the lowest penalty, and is therefore expected to
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* maximize fan-out. Sets *newitemonleft for us.
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*/
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foundfirstright = _bt_bestsplitloc(&state, perfectpenalty, newitemonleft);
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pfree(state.splits);
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return foundfirstright;
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}
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/*
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* Subroutine to record a particular point between two tuples (possibly the
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* new item) on page (ie, combination of firstright and newitemonleft
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* settings) in *state for later analysis. This is also a convenient point
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* to check if the split is legal (if it isn't, it won't be recorded).
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*
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* firstoldonright is the offset of the first item on the original page that
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* goes to the right page, and firstoldonrightsz is the size of that tuple.
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* firstoldonright can be > max offset, which means that all the old items go
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* to the left page and only the new item goes to the right page. In that
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* case, firstoldonrightsz is not used.
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*
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* olddataitemstoleft is the total size of all old items to the left of the
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* split point that is recorded here when legal. Should not include
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* newitemsz, since that is handled here.
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*/
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static void
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_bt_recsplitloc(FindSplitData *state,
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OffsetNumber firstoldonright,
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bool newitemonleft,
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int olddataitemstoleft,
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Size firstoldonrightsz)
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{
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int16 leftfree,
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rightfree;
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Size firstrightitemsz;
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|
bool newitemisfirstonright;
|
||
|
|
||
|
/* Is the new item going to be the first item on the right page? */
|
||
|
newitemisfirstonright = (firstoldonright == state->newitemoff
|
||
|
&& !newitemonleft);
|
||
|
|
||
|
if (newitemisfirstonright)
|
||
|
firstrightitemsz = state->newitemsz;
|
||
|
else
|
||
|
firstrightitemsz = firstoldonrightsz;
|
||
|
|
||
|
/* Account for all the old tuples */
|
||
|
leftfree = state->leftspace - olddataitemstoleft;
|
||
|
rightfree = state->rightspace -
|
||
|
(state->olddataitemstotal - olddataitemstoleft);
|
||
|
|
||
|
/*
|
||
|
* The first item on the right page becomes the high key of the left page;
|
||
|
* therefore it counts against left space as well as right space (we
|
||
|
* cannot assume that suffix truncation will make it any smaller). When
|
||
|
* index has included attributes, then those attributes of left page high
|
||
|
* key will be truncated leaving that page with slightly more free space.
|
||
|
* However, that shouldn't affect our ability to find valid split
|
||
|
* location, since we err in the direction of being pessimistic about free
|
||
|
* space on the left half. Besides, even when suffix truncation of
|
||
|
* non-TID attributes occurs, the new high key often won't even be a
|
||
|
* single MAXALIGN() quantum smaller than the firstright tuple it's based
|
||
|
* on.
|
||
|
*
|
||
|
* If we are on the leaf level, assume that suffix truncation cannot avoid
|
||
|
* adding a heap TID to the left half's new high key when splitting at the
|
||
|
* leaf level. In practice the new high key will often be smaller and
|
||
|
* will rarely be larger, but conservatively assume the worst case.
|
||
|
*/
|
||
|
if (state->is_leaf)
|
||
|
leftfree -= (int16) (firstrightitemsz +
|
||
|
MAXALIGN(sizeof(ItemPointerData)));
|
||
|
else
|
||
|
leftfree -= (int16) firstrightitemsz;
|
||
|
|
||
|
/* account for the new item */
|
||
|
if (newitemonleft)
|
||
|
leftfree -= (int16) state->newitemsz;
|
||
|
else
|
||
|
rightfree -= (int16) state->newitemsz;
|
||
|
|
||
|
/*
|
||
|
* If we are not on the leaf level, we will be able to discard the key
|
||
|
* data from the first item that winds up on the right page.
|
||
|
*/
|
||
|
if (!state->is_leaf)
|
||
|
rightfree += (int16) firstrightitemsz -
|
||
|
(int16) (MAXALIGN(sizeof(IndexTupleData)) + sizeof(ItemIdData));
|
||
|
|
||
|
/* Record split if legal */
|
||
|
if (leftfree >= 0 && rightfree >= 0)
|
||
|
{
|
||
|
Assert(state->nsplits < state->maxsplits);
|
||
|
|
||
|
/* Determine smallest firstright item size on page */
|
||
|
state->minfirstrightsz = Min(state->minfirstrightsz, firstrightitemsz);
|
||
|
|
||
|
state->splits[state->nsplits].curdelta = 0;
|
||
|
state->splits[state->nsplits].leftfree = leftfree;
|
||
|
state->splits[state->nsplits].rightfree = rightfree;
|
||
|
state->splits[state->nsplits].firstoldonright = firstoldonright;
|
||
|
state->splits[state->nsplits].newitemonleft = newitemonleft;
|
||
|
state->nsplits++;
|
||
|
}
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to assign space deltas to materialized array of candidate split
|
||
|
* points based on current fillfactor, and to sort array using that fillfactor
|
||
|
*/
|
||
|
static void
|
||
|
_bt_deltasortsplits(FindSplitData *state, double fillfactormult,
|
||
|
bool usemult)
|
||
|
{
|
||
|
for (int i = 0; i < state->nsplits; i++)
|
||
|
{
|
||
|
SplitPoint *split = state->splits + i;
|
||
|
int16 delta;
|
||
|
|
||
|
if (usemult)
|
||
|
delta = fillfactormult * split->leftfree -
|
||
|
(1.0 - fillfactormult) * split->rightfree;
|
||
|
else
|
||
|
delta = split->leftfree - split->rightfree;
|
||
|
|
||
|
if (delta < 0)
|
||
|
delta = -delta;
|
||
|
|
||
|
/* Save delta */
|
||
|
split->curdelta = delta;
|
||
|
}
|
||
|
|
||
|
qsort(state->splits, state->nsplits, sizeof(SplitPoint), _bt_splitcmp);
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* qsort-style comparator used by _bt_deltasortsplits()
|
||
|
*/
|
||
|
static int
|
||
|
_bt_splitcmp(const void *arg1, const void *arg2)
|
||
|
{
|
||
|
SplitPoint *split1 = (SplitPoint *) arg1;
|
||
|
SplitPoint *split2 = (SplitPoint *) arg2;
|
||
|
|
||
|
if (split1->curdelta > split2->curdelta)
|
||
|
return 1;
|
||
|
if (split1->curdelta < split2->curdelta)
|
||
|
return -1;
|
||
|
|
||
|
return 0;
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to find the "best" split point among an array of acceptable
|
||
|
* candidate split points that split without there being an excessively high
|
||
|
* delta between the space left free on the left and right halves. The "best"
|
||
|
* split point is the split point with the lowest penalty among split points
|
||
|
* that fall within current/final split interval. Penalty is an abstract
|
||
|
* score, with a definition that varies depending on whether we're splitting a
|
||
|
* leaf page or an internal page. See _bt_split_penalty() for details.
|
||
|
*
|
||
|
* "perfectpenalty" is assumed to be the lowest possible penalty among
|
||
|
* candidate split points. This allows us to return early without wasting
|
||
|
* cycles on calculating the first differing attribute for all candidate
|
||
|
* splits when that clearly cannot improve our choice (or when we only want a
|
||
|
* minimally distinguishing split point, and don't want to make the split any
|
||
|
* more unbalanced than is necessary).
|
||
|
*
|
||
|
* We return the index of the first existing tuple that should go on the right
|
||
|
* page, plus a boolean indicating if new item is on left of split point.
|
||
|
*/
|
||
|
static OffsetNumber
|
||
|
_bt_bestsplitloc(FindSplitData *state, int perfectpenalty, bool *newitemonleft)
|
||
|
{
|
||
|
int bestpenalty,
|
||
|
lowsplit;
|
||
|
int highsplit = Min(state->interval, state->nsplits);
|
||
|
|
||
|
/* No point in calculating penalty when there's only one choice */
|
||
|
if (state->nsplits == 1)
|
||
|
{
|
||
|
*newitemonleft = state->splits[0].newitemonleft;
|
||
|
return state->splits[0].firstoldonright;
|
||
|
}
|
||
|
|
||
|
bestpenalty = INT_MAX;
|
||
|
lowsplit = 0;
|
||
|
for (int i = lowsplit; i < highsplit; i++)
|
||
|
{
|
||
|
int penalty;
|
||
|
|
||
|
penalty = _bt_split_penalty(state, state->splits + i);
|
||
|
|
||
|
if (penalty <= perfectpenalty)
|
||
|
{
|
||
|
bestpenalty = penalty;
|
||
|
lowsplit = i;
|
||
|
break;
|
||
|
}
|
||
|
|
||
|
if (penalty < bestpenalty)
|
||
|
{
|
||
|
bestpenalty = penalty;
|
||
|
lowsplit = i;
|
||
|
}
|
||
|
}
|
||
|
|
||
|
*newitemonleft = state->splits[lowsplit].newitemonleft;
|
||
|
return state->splits[lowsplit].firstoldonright;
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to decide whether split should use default strategy/initial
|
||
|
* split interval, or whether it should finish splitting the page using
|
||
|
* alternative strategies (this is only possible with leaf pages).
|
||
|
*
|
||
|
* Caller uses alternative strategy (or sticks with default strategy) based
|
||
|
* on how *strategy is set here. Return value is "perfect penalty", which is
|
||
|
* passed to _bt_bestsplitloc() as a final constraint on how far caller is
|
||
|
* willing to go to avoid appending a heap TID when using the many duplicates
|
||
|
* strategy (it also saves _bt_bestsplitloc() useless cycles).
|
||
|
*/
|
||
|
static int
|
||
|
_bt_strategy(FindSplitData *state, SplitPoint *leftpage,
|
||
|
SplitPoint *rightpage, FindSplitStrat *strategy)
|
||
|
{
|
||
|
IndexTuple leftmost,
|
||
|
rightmost;
|
||
|
SplitPoint *leftinterval,
|
||
|
*rightinterval;
|
||
|
int perfectpenalty;
|
||
|
int indnkeyatts = IndexRelationGetNumberOfKeyAttributes(state->rel);
|
||
|
|
||
|
/* Assume that alternative strategy won't be used for now */
|
||
|
*strategy = SPLIT_DEFAULT;
|
||
|
|
||
|
/*
|
||
|
* Use smallest observed first right item size for entire page as perfect
|
||
|
* penalty on internal pages. This can save cycles in the common case
|
||
|
* where most or all splits (not just splits within interval) have first
|
||
|
* right tuples that are the same size.
|
||
|
*/
|
||
|
if (!state->is_leaf)
|
||
|
return state->minfirstrightsz;
|
||
|
|
||
|
/*
|
||
|
* Use leftmost and rightmost tuples from leftmost and rightmost splits in
|
||
|
* current split interval
|
||
|
*/
|
||
|
_bt_interval_edges(state, &leftinterval, &rightinterval);
|
||
|
leftmost = _bt_split_lastleft(state, leftinterval);
|
||
|
rightmost = _bt_split_firstright(state, rightinterval);
|
||
|
|
||
|
/*
|
||
|
* If initial split interval can produce a split point that will at least
|
||
|
* avoid appending a heap TID in new high key, we're done. Finish split
|
||
|
* with default strategy and initial split interval.
|
||
|
*/
|
||
|
perfectpenalty = _bt_keep_natts_fast(state->rel, leftmost, rightmost);
|
||
|
if (perfectpenalty <= indnkeyatts)
|
||
|
return perfectpenalty;
|
||
|
|
||
|
/*
|
||
|
* Work out how caller should finish split when even their "perfect"
|
||
|
* penalty for initial/default split interval indicates that the interval
|
||
|
* does not contain even a single split that avoids appending a heap TID.
|
||
|
*
|
||
|
* Use the leftmost split's lastleft tuple and the rightmost split's
|
||
|
* firstright tuple to assess every possible split.
|
||
|
*/
|
||
|
leftmost = _bt_split_lastleft(state, leftpage);
|
||
|
rightmost = _bt_split_firstright(state, rightpage);
|
||
|
|
||
|
/*
|
||
|
* If page (including new item) has many duplicates but is not entirely
|
||
|
* full of duplicates, a many duplicates strategy split will be performed.
|
||
|
* If page is entirely full of duplicates, a single value strategy split
|
||
|
* will be performed.
|
||
|
*/
|
||
|
perfectpenalty = _bt_keep_natts_fast(state->rel, leftmost, rightmost);
|
||
|
if (perfectpenalty <= indnkeyatts)
|
||
|
{
|
||
|
*strategy = SPLIT_MANY_DUPLICATES;
|
||
|
|
||
|
/*
|
||
|
* Caller should choose the lowest delta split that avoids appending a
|
||
|
* heap TID. Maximizing the number of attributes that can be
|
||
|
* truncated away (returning perfectpenalty when it happens to be less
|
||
|
* than the number of key attributes in index) can result in continual
|
||
|
* unbalanced page splits.
|
||
|
*
|
||
|
* Just avoiding appending a heap TID can still make splits very
|
||
|
* unbalanced, but this is self-limiting. When final split has a very
|
||
|
* high delta, one side of the split will likely consist of a single
|
||
|
* value. If that page is split once again, then that split will
|
||
|
* likely use the single value strategy.
|
||
|
*/
|
||
|
return indnkeyatts;
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Single value strategy is only appropriate with ever-increasing heap
|
||
|
* TIDs; otherwise, original default strategy split should proceed to
|
||
|
* avoid pathological performance. Use page high key to infer if this is
|
||
|
* the rightmost page among pages that store the same duplicate value.
|
||
|
* This should not prevent insertions of heap TIDs that are slightly out
|
||
|
* of order from using single value strategy, since that's expected with
|
||
|
* concurrent inserters of the same duplicate value.
|
||
|
*/
|
||
|
else if (state->is_rightmost)
|
||
|
*strategy = SPLIT_SINGLE_VALUE;
|
||
|
else
|
||
|
{
|
||
|
ItemId itemid;
|
||
|
IndexTuple hikey;
|
||
|
|
||
|
itemid = PageGetItemId(state->page, P_HIKEY);
|
||
|
hikey = (IndexTuple) PageGetItem(state->page, itemid);
|
||
|
perfectpenalty = _bt_keep_natts_fast(state->rel, hikey,
|
||
|
state->newitem);
|
||
|
if (perfectpenalty <= indnkeyatts)
|
||
|
*strategy = SPLIT_SINGLE_VALUE;
|
||
|
else
|
||
|
{
|
||
|
/*
|
||
|
* Have caller finish split using default strategy, since page
|
||
|
* does not appear to be the rightmost page for duplicates of the
|
||
|
* value the page is filled with
|
||
|
*/
|
||
|
}
|
||
|
}
|
||
|
|
||
|
return perfectpenalty;
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to locate leftmost and rightmost splits for current/default
|
||
|
* split interval. Note that it will be the same split iff there is only one
|
||
|
* split in interval.
|
||
|
*/
|
||
|
static void
|
||
|
_bt_interval_edges(FindSplitData *state, SplitPoint **leftinterval,
|
||
|
SplitPoint **rightinterval)
|
||
|
{
|
||
|
int highsplit = Min(state->interval, state->nsplits);
|
||
|
SplitPoint *deltaoptimal;
|
||
|
|
||
|
deltaoptimal = state->splits;
|
||
|
*leftinterval = NULL;
|
||
|
*rightinterval = NULL;
|
||
|
|
||
|
/*
|
||
|
* Delta is an absolute distance to optimal split point, so both the
|
||
|
* leftmost and rightmost split point will usually be at the end of the
|
||
|
* array
|
||
|
*/
|
||
|
for (int i = highsplit - 1; i >= 0; i--)
|
||
|
{
|
||
|
SplitPoint *distant = state->splits + i;
|
||
|
|
||
|
if (distant->firstoldonright < deltaoptimal->firstoldonright)
|
||
|
{
|
||
|
if (*leftinterval == NULL)
|
||
|
*leftinterval = distant;
|
||
|
}
|
||
|
else if (distant->firstoldonright > deltaoptimal->firstoldonright)
|
||
|
{
|
||
|
if (*rightinterval == NULL)
|
||
|
*rightinterval = distant;
|
||
|
}
|
||
|
else if (!distant->newitemonleft && deltaoptimal->newitemonleft)
|
||
|
{
|
||
|
/*
|
||
|
* "incoming tuple will become first on right page" (distant) is
|
||
|
* to the left of "incoming tuple will become last on left page"
|
||
|
* (delta-optimal)
|
||
|
*/
|
||
|
Assert(distant->firstoldonright == state->newitemoff);
|
||
|
if (*leftinterval == NULL)
|
||
|
*leftinterval = distant;
|
||
|
}
|
||
|
else if (distant->newitemonleft && !deltaoptimal->newitemonleft)
|
||
|
{
|
||
|
/*
|
||
|
* "incoming tuple will become last on left page" (distant) is to
|
||
|
* the right of "incoming tuple will become first on right page"
|
||
|
* (delta-optimal)
|
||
|
*/
|
||
|
Assert(distant->firstoldonright == state->newitemoff);
|
||
|
if (*rightinterval == NULL)
|
||
|
*rightinterval = distant;
|
||
|
}
|
||
|
else
|
||
|
{
|
||
|
/* There was only one or two splits in initial split interval */
|
||
|
Assert(distant == deltaoptimal);
|
||
|
if (*leftinterval == NULL)
|
||
|
*leftinterval = distant;
|
||
|
if (*rightinterval == NULL)
|
||
|
*rightinterval = distant;
|
||
|
}
|
||
|
|
||
|
if (*leftinterval && *rightinterval)
|
||
|
return;
|
||
|
}
|
||
|
|
||
|
Assert(false);
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to find penalty for caller's candidate split point.
|
||
|
*
|
||
|
* On leaf pages, penalty is the attribute number that distinguishes each side
|
||
|
* of a split. It's the last attribute that needs to be included in new high
|
||
|
* key for left page. It can be greater than the number of key attributes in
|
||
|
* cases where a heap TID will need to be appended during truncation.
|
||
|
*
|
||
|
* On internal pages, penalty is simply the size of the first item on the
|
||
|
* right half of the split (including line pointer overhead). This tuple will
|
||
|
* become the new high key for the left page.
|
||
|
*/
|
||
|
static inline int
|
||
|
_bt_split_penalty(FindSplitData *state, SplitPoint *split)
|
||
|
{
|
||
|
IndexTuple lastleftuple;
|
||
|
IndexTuple firstrighttuple;
|
||
|
|
||
|
if (!state->is_leaf)
|
||
|
{
|
||
|
ItemId itemid;
|
||
|
|
||
|
if (!split->newitemonleft &&
|
||
|
split->firstoldonright == state->newitemoff)
|
||
|
return state->newitemsz;
|
||
|
|
||
|
itemid = PageGetItemId(state->page, split->firstoldonright);
|
||
|
|
||
|
return MAXALIGN(ItemIdGetLength(itemid)) + sizeof(ItemIdData);
|
||
|
}
|
||
|
|
||
|
lastleftuple = _bt_split_lastleft(state, split);
|
||
|
firstrighttuple = _bt_split_firstright(state, split);
|
||
|
|
||
|
Assert(lastleftuple != firstrighttuple);
|
||
|
return _bt_keep_natts_fast(state->rel, lastleftuple, firstrighttuple);
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to get a lastleft IndexTuple for a spit point from page
|
||
|
*/
|
||
|
static inline IndexTuple
|
||
|
_bt_split_lastleft(FindSplitData *state, SplitPoint *split)
|
||
|
{
|
||
|
ItemId itemid;
|
||
|
|
||
|
if (split->newitemonleft && split->firstoldonright == state->newitemoff)
|
||
|
return state->newitem;
|
||
|
|
||
|
itemid = PageGetItemId(state->page,
|
||
|
OffsetNumberPrev(split->firstoldonright));
|
||
|
return (IndexTuple) PageGetItem(state->page, itemid);
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Subroutine to get a firstright IndexTuple for a spit point from page
|
||
|
*/
|
||
|
static inline IndexTuple
|
||
|
_bt_split_firstright(FindSplitData *state, SplitPoint *split)
|
||
|
{
|
||
|
ItemId itemid;
|
||
|
|
||
|
if (!split->newitemonleft && split->firstoldonright == state->newitemoff)
|
||
|
return state->newitem;
|
||
|
|
||
|
itemid = PageGetItemId(state->page, split->firstoldonright);
|
||
|
return (IndexTuple) PageGetItem(state->page, itemid);
|
||
|
}
|