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Diffstat (limited to 'src/backend/access/nbtree/nbtdedup.c')
| -rw-r--r-- | src/backend/access/nbtree/nbtdedup.c | 857 |
1 files changed, 857 insertions, 0 deletions
diff --git a/src/backend/access/nbtree/nbtdedup.c b/src/backend/access/nbtree/nbtdedup.c new file mode 100644 index 0000000..cb53ce4 --- /dev/null +++ b/src/backend/access/nbtree/nbtdedup.c @@ -0,0 +1,857 @@ +/*------------------------------------------------------------------------- + * + * nbtdedup.c + * Deduplicate items in Postgres btrees. + * + * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group + * Portions Copyright (c) 1994, Regents of the University of California + * + * + * IDENTIFICATION + * src/backend/access/nbtree/nbtdedup.c + * + *------------------------------------------------------------------------- + */ +#include "postgres.h" + +#include "access/nbtree.h" +#include "access/nbtxlog.h" +#include "miscadmin.h" +#include "utils/rel.h" + +static bool _bt_do_singleval(Relation rel, Page page, BTDedupState state, + OffsetNumber minoff, IndexTuple newitem); +static void _bt_singleval_fillfactor(Page page, BTDedupState state, + Size newitemsz); +#ifdef USE_ASSERT_CHECKING +static bool _bt_posting_valid(IndexTuple posting); +#endif + +/* + * Deduplicate items on a leaf page. The page will have to be split by caller + * if we cannot successfully free at least newitemsz (we also need space for + * newitem's line pointer, which isn't included in caller's newitemsz). + * + * The general approach taken here is to perform as much deduplication as + * possible to free as much space as possible. Note, however, that "single + * value" strategy is sometimes used for !checkingunique callers, in which + * case deduplication will leave a few tuples untouched at the end of the + * page. The general idea is to prepare the page for an anticipated page + * split that uses nbtsplitloc.c's "single value" strategy to determine a + * split point. (There is no reason to deduplicate items that will end up on + * the right half of the page after the anticipated page split; better to + * handle those if and when the anticipated right half page gets its own + * deduplication pass, following further inserts of duplicates.) + * + * This function should be called during insertion, when the page doesn't have + * enough space to fit an incoming newitem. If the BTP_HAS_GARBAGE page flag + * was set, caller should have removed any LP_DEAD items by calling + * _bt_vacuum_one_page() before calling here. We may still have to kill + * LP_DEAD items here when the page's BTP_HAS_GARBAGE hint is falsely unset, + * but that should be rare. Also, _bt_vacuum_one_page() won't unset the + * BTP_HAS_GARBAGE flag when it finds no LP_DEAD items, so a successful + * deduplication pass will always clear it, just to keep things tidy. + */ +void +_bt_dedup_one_page(Relation rel, Buffer buf, Relation heapRel, + IndexTuple newitem, Size newitemsz, bool checkingunique) +{ + OffsetNumber offnum, + minoff, + maxoff; + Page page = BufferGetPage(buf); + BTPageOpaque opaque; + Page newpage; + OffsetNumber deletable[MaxIndexTuplesPerPage]; + BTDedupState state; + int ndeletable = 0; + Size pagesaving = 0; + bool singlevalstrat = false; + int nkeyatts = IndexRelationGetNumberOfKeyAttributes(rel); + + /* + * We can't assume that there are no LP_DEAD items. For one thing, VACUUM + * will clear the BTP_HAS_GARBAGE hint without reliably removing items + * that are marked LP_DEAD. We don't want to unnecessarily unset LP_DEAD + * bits when deduplicating items. Allowing it would be correct, though + * wasteful. + */ + opaque = (BTPageOpaque) PageGetSpecialPointer(page); + minoff = P_FIRSTDATAKEY(opaque); + maxoff = PageGetMaxOffsetNumber(page); + for (offnum = minoff; + offnum <= maxoff; + offnum = OffsetNumberNext(offnum)) + { + ItemId itemid = PageGetItemId(page, offnum); + + if (ItemIdIsDead(itemid)) + deletable[ndeletable++] = offnum; + } + + if (ndeletable > 0) + { + _bt_delitems_delete(rel, buf, deletable, ndeletable, heapRel); + + /* + * Return when a split will be avoided. This is equivalent to + * avoiding a split using the usual _bt_vacuum_one_page() path. + */ + if (PageGetFreeSpace(page) >= newitemsz) + return; + + /* + * Reconsider number of items on page, in case _bt_delitems_delete() + * managed to delete an item or two + */ + minoff = P_FIRSTDATAKEY(opaque); + maxoff = PageGetMaxOffsetNumber(page); + } + + /* Passed-in newitemsz is MAXALIGNED but does not include line pointer */ + newitemsz += sizeof(ItemIdData); + + /* + * By here, it's clear that deduplication will definitely be attempted. + * Initialize deduplication state. + * + * It would be possible for maxpostingsize (limit on posting list tuple + * size) to be set to one third of the page. However, it seems like a + * good idea to limit the size of posting lists to one sixth of a page. + * That ought to leave us with a good split point when pages full of + * duplicates can be split several times. + */ + state = (BTDedupState) palloc(sizeof(BTDedupStateData)); + state->deduplicate = true; + state->nmaxitems = 0; + state->maxpostingsize = Min(BTMaxItemSize(page) / 2, INDEX_SIZE_MASK); + /* Metadata about base tuple of current pending posting list */ + state->base = NULL; + state->baseoff = InvalidOffsetNumber; + state->basetupsize = 0; + /* Metadata about current pending posting list TIDs */ + state->htids = palloc(state->maxpostingsize); + state->nhtids = 0; + state->nitems = 0; + /* Size of all physical tuples to be replaced by pending posting list */ + state->phystupsize = 0; + /* nintervals should be initialized to zero */ + state->nintervals = 0; + + /* Determine if "single value" strategy should be used */ + if (!checkingunique) + singlevalstrat = _bt_do_singleval(rel, page, state, minoff, newitem); + + /* + * Deduplicate items from page, and write them to newpage. + * + * Copy the original page's LSN into newpage copy. This will become the + * updated version of the page. We need this because XLogInsert will + * examine the LSN and possibly dump it in a page image. + */ + newpage = PageGetTempPageCopySpecial(page); + PageSetLSN(newpage, PageGetLSN(page)); + + /* Copy high key, if any */ + if (!P_RIGHTMOST(opaque)) + { + ItemId hitemid = PageGetItemId(page, P_HIKEY); + Size hitemsz = ItemIdGetLength(hitemid); + IndexTuple hitem = (IndexTuple) PageGetItem(page, hitemid); + + if (PageAddItem(newpage, (Item) hitem, hitemsz, P_HIKEY, + false, false) == InvalidOffsetNumber) + elog(ERROR, "deduplication failed to add highkey"); + } + + for (offnum = minoff; + offnum <= maxoff; + offnum = OffsetNumberNext(offnum)) + { + ItemId itemid = PageGetItemId(page, offnum); + IndexTuple itup = (IndexTuple) PageGetItem(page, itemid); + + Assert(!ItemIdIsDead(itemid)); + + if (offnum == minoff) + { + /* + * No previous/base tuple for the data item -- use the data item + * as base tuple of pending posting list + */ + _bt_dedup_start_pending(state, itup, offnum); + } + else if (state->deduplicate && + _bt_keep_natts_fast(rel, state->base, itup) > nkeyatts && + _bt_dedup_save_htid(state, itup)) + { + /* + * Tuple is equal to base tuple of pending posting list. Heap + * TID(s) for itup have been saved in state. + */ + } + else + { + /* + * Tuple is not equal to pending posting list tuple, or + * _bt_dedup_save_htid() opted to not merge current item into + * pending posting list for some other reason (e.g., adding more + * TIDs would have caused posting list to exceed current + * maxpostingsize). + * + * If state contains pending posting list with more than one item, + * form new posting tuple, and actually update the page. Else + * reset the state and move on without modifying the page. + */ + pagesaving += _bt_dedup_finish_pending(newpage, state); + + if (singlevalstrat) + { + /* + * Single value strategy's extra steps. + * + * Lower maxpostingsize for sixth and final large posting list + * tuple at the point where 5 maxpostingsize-capped tuples + * have either been formed or observed. + * + * When a sixth maxpostingsize-capped item is formed/observed, + * stop merging together tuples altogether. The few tuples + * that remain at the end of the page won't be merged together + * at all (at least not until after a future page split takes + * place). + */ + if (state->nmaxitems == 5) + _bt_singleval_fillfactor(page, state, newitemsz); + else if (state->nmaxitems == 6) + { + state->deduplicate = false; + singlevalstrat = false; /* won't be back here */ + } + } + + /* itup starts new pending posting list */ + _bt_dedup_start_pending(state, itup, offnum); + } + } + + /* Handle the last item */ + pagesaving += _bt_dedup_finish_pending(newpage, state); + + /* + * If no items suitable for deduplication were found, newpage must be + * exactly the same as the original page, so just return from function. + * + * We could determine whether or not to proceed on the basis the space + * savings being sufficient to avoid an immediate page split instead. We + * don't do that because there is some small value in nbtsplitloc.c always + * operating against a page that is fully deduplicated (apart from + * newitem). Besides, most of the cost has already been paid. + */ + if (state->nintervals == 0) + { + /* cannot leak memory here */ + pfree(newpage); + pfree(state->htids); + pfree(state); + return; + } + + /* + * By here, it's clear that deduplication will definitely go ahead. + * + * Clear the BTP_HAS_GARBAGE page flag in the unlikely event that it is + * still falsely set, just to keep things tidy. (We can't rely on + * _bt_vacuum_one_page() having done this already, and we can't rely on a + * page split or VACUUM getting to it in the near future.) + */ + if (P_HAS_GARBAGE(opaque)) + { + BTPageOpaque nopaque = (BTPageOpaque) PageGetSpecialPointer(newpage); + + nopaque->btpo_flags &= ~BTP_HAS_GARBAGE; + } + + START_CRIT_SECTION(); + + PageRestoreTempPage(newpage, page); + MarkBufferDirty(buf); + + /* XLOG stuff */ + if (RelationNeedsWAL(rel)) + { + XLogRecPtr recptr; + xl_btree_dedup xlrec_dedup; + + xlrec_dedup.nintervals = state->nintervals; + + XLogBeginInsert(); + XLogRegisterBuffer(0, buf, REGBUF_STANDARD); + XLogRegisterData((char *) &xlrec_dedup, SizeOfBtreeDedup); + + /* + * The intervals array is not in the buffer, but pretend that it is. + * When XLogInsert stores the whole buffer, the array need not be + * stored too. + */ + XLogRegisterBufData(0, (char *) state->intervals, + state->nintervals * sizeof(BTDedupInterval)); + + recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_DEDUP); + + PageSetLSN(page, recptr); + } + + END_CRIT_SECTION(); + + /* Local space accounting should agree with page accounting */ + Assert(pagesaving < newitemsz || PageGetExactFreeSpace(page) >= newitemsz); + + /* cannot leak memory here */ + pfree(state->htids); + pfree(state); +} + +/* + * Create a new pending posting list tuple based on caller's base tuple. + * + * Every tuple processed by deduplication either becomes the base tuple for a + * posting list, or gets its heap TID(s) accepted into a pending posting list. + * A tuple that starts out as the base tuple for a posting list will only + * actually be rewritten within _bt_dedup_finish_pending() when it turns out + * that there are duplicates that can be merged into the base tuple. + */ +void +_bt_dedup_start_pending(BTDedupState state, IndexTuple base, + OffsetNumber baseoff) +{ + Assert(state->nhtids == 0); + Assert(state->nitems == 0); + Assert(!BTreeTupleIsPivot(base)); + + /* + * Copy heap TID(s) from new base tuple for new candidate posting list + * into working state's array + */ + if (!BTreeTupleIsPosting(base)) + { + memcpy(state->htids, &base->t_tid, sizeof(ItemPointerData)); + state->nhtids = 1; + state->basetupsize = IndexTupleSize(base); + } + else + { + int nposting; + + nposting = BTreeTupleGetNPosting(base); + memcpy(state->htids, BTreeTupleGetPosting(base), + sizeof(ItemPointerData) * nposting); + state->nhtids = nposting; + /* basetupsize should not include existing posting list */ + state->basetupsize = BTreeTupleGetPostingOffset(base); + } + + /* + * Save new base tuple itself -- it'll be needed if we actually create a + * new posting list from new pending posting list. + * + * Must maintain physical size of all existing tuples (including line + * pointer overhead) so that we can calculate space savings on page. + */ + state->nitems = 1; + state->base = base; + state->baseoff = baseoff; + state->phystupsize = MAXALIGN(IndexTupleSize(base)) + sizeof(ItemIdData); + /* Also save baseoff in pending state for interval */ + state->intervals[state->nintervals].baseoff = state->baseoff; +} + +/* + * Save itup heap TID(s) into pending posting list where possible. + * + * Returns bool indicating if the pending posting list managed by state now + * includes itup's heap TID(s). + */ +bool +_bt_dedup_save_htid(BTDedupState state, IndexTuple itup) +{ + int nhtids; + ItemPointer htids; + Size mergedtupsz; + + Assert(!BTreeTupleIsPivot(itup)); + + if (!BTreeTupleIsPosting(itup)) + { + nhtids = 1; + htids = &itup->t_tid; + } + else + { + nhtids = BTreeTupleGetNPosting(itup); + htids = BTreeTupleGetPosting(itup); + } + + /* + * Don't append (have caller finish pending posting list as-is) if + * appending heap TID(s) from itup would put us over maxpostingsize limit. + * + * This calculation needs to match the code used within _bt_form_posting() + * for new posting list tuples. + */ + mergedtupsz = MAXALIGN(state->basetupsize + + (state->nhtids + nhtids) * sizeof(ItemPointerData)); + + if (mergedtupsz > state->maxpostingsize) + { + /* + * Count this as an oversized item for single value strategy, though + * only when there are 50 TIDs in the final posting list tuple. This + * limit (which is fairly arbitrary) avoids confusion about how many + * 1/6 of a page tuples have been encountered/created by the current + * deduplication pass. + * + * Note: We deliberately don't consider which deduplication pass + * merged together tuples to create this item (could be a previous + * deduplication pass, or current pass). See _bt_do_singleval() + * comments. + */ + if (state->nhtids > 50) + state->nmaxitems++; + + return false; + } + + /* + * Save heap TIDs to pending posting list tuple -- itup can be merged into + * pending posting list + */ + state->nitems++; + memcpy(state->htids + state->nhtids, htids, + sizeof(ItemPointerData) * nhtids); + state->nhtids += nhtids; + state->phystupsize += MAXALIGN(IndexTupleSize(itup)) + sizeof(ItemIdData); + + return true; +} + +/* + * Finalize pending posting list tuple, and add it to the page. Final tuple + * is based on saved base tuple, and saved list of heap TIDs. + * + * Returns space saving from deduplicating to make a new posting list tuple. + * Note that this includes line pointer overhead. This is zero in the case + * where no deduplication was possible. + */ +Size +_bt_dedup_finish_pending(Page newpage, BTDedupState state) +{ + OffsetNumber tupoff; + Size tuplesz; + Size spacesaving; + + Assert(state->nitems > 0); + Assert(state->nitems <= state->nhtids); + Assert(state->intervals[state->nintervals].baseoff == state->baseoff); + + tupoff = OffsetNumberNext(PageGetMaxOffsetNumber(newpage)); + if (state->nitems == 1) + { + /* Use original, unchanged base tuple */ + tuplesz = IndexTupleSize(state->base); + if (PageAddItem(newpage, (Item) state->base, tuplesz, tupoff, + false, false) == InvalidOffsetNumber) + elog(ERROR, "deduplication failed to add tuple to page"); + + spacesaving = 0; + } + else + { + IndexTuple final; + + /* Form a tuple with a posting list */ + final = _bt_form_posting(state->base, state->htids, state->nhtids); + tuplesz = IndexTupleSize(final); + Assert(tuplesz <= state->maxpostingsize); + + /* Save final number of items for posting list */ + state->intervals[state->nintervals].nitems = state->nitems; + + Assert(tuplesz == MAXALIGN(IndexTupleSize(final))); + if (PageAddItem(newpage, (Item) final, tuplesz, tupoff, false, + false) == InvalidOffsetNumber) + elog(ERROR, "deduplication failed to add tuple to page"); + + pfree(final); + spacesaving = state->phystupsize - (tuplesz + sizeof(ItemIdData)); + /* Increment nintervals, since we wrote a new posting list tuple */ + state->nintervals++; + Assert(spacesaving > 0 && spacesaving < BLCKSZ); + } + + /* Reset state for next pending posting list */ + state->nhtids = 0; + state->nitems = 0; + state->phystupsize = 0; + + return spacesaving; +} + +/* + * Determine if page non-pivot tuples (data items) are all duplicates of the + * same value -- if they are, deduplication's "single value" strategy should + * be applied. The general goal of this strategy is to ensure that + * nbtsplitloc.c (which uses its own single value strategy) will find a useful + * split point as further duplicates are inserted, and successive rightmost + * page splits occur among pages that store the same duplicate value. When + * the page finally splits, it should end up BTREE_SINGLEVAL_FILLFACTOR% full, + * just like it would if deduplication were disabled. + * + * We expect that affected workloads will require _several_ single value + * strategy deduplication passes (over a page that only stores duplicates) + * before the page is finally split. The first deduplication pass should only + * find regular non-pivot tuples. Later deduplication passes will find + * existing maxpostingsize-capped posting list tuples, which must be skipped + * over. The penultimate pass is generally the first pass that actually + * reaches _bt_singleval_fillfactor(), and so will deliberately leave behind a + * few untouched non-pivot tuples. The final deduplication pass won't free + * any space -- it will skip over everything without merging anything (it + * retraces the steps of the penultimate pass). + * + * Fortunately, having several passes isn't too expensive. Each pass (after + * the first pass) won't spend many cycles on the large posting list tuples + * left by previous passes. Each pass will find a large contiguous group of + * smaller duplicate tuples to merge together at the end of the page. + * + * Note: We deliberately don't bother checking if the high key is a distinct + * value (prior to the TID tiebreaker column) before proceeding, unlike + * nbtsplitloc.c. Its single value strategy only gets applied on the + * rightmost page of duplicates of the same value (other leaf pages full of + * duplicates will get a simple 50:50 page split instead of splitting towards + * the end of the page). There is little point in making the same distinction + * here. + */ +static bool +_bt_do_singleval(Relation rel, Page page, BTDedupState state, + OffsetNumber minoff, IndexTuple newitem) +{ + int nkeyatts = IndexRelationGetNumberOfKeyAttributes(rel); + ItemId itemid; + IndexTuple itup; + + itemid = PageGetItemId(page, minoff); + itup = (IndexTuple) PageGetItem(page, itemid); + + if (_bt_keep_natts_fast(rel, newitem, itup) > nkeyatts) + { + itemid = PageGetItemId(page, PageGetMaxOffsetNumber(page)); + itup = (IndexTuple) PageGetItem(page, itemid); + + if (_bt_keep_natts_fast(rel, newitem, itup) > nkeyatts) + return true; + } + + return false; +} + +/* + * Lower maxpostingsize when using "single value" strategy, to avoid a sixth + * and final maxpostingsize-capped tuple. The sixth and final posting list + * tuple will end up somewhat smaller than the first five. (Note: The first + * five tuples could actually just be very large duplicate tuples that + * couldn't be merged together at all. Deduplication will simply not modify + * the page when that happens.) + * + * When there are six posting lists on the page (after current deduplication + * pass goes on to create/observe a sixth very large tuple), caller should end + * its deduplication pass. It isn't useful to try to deduplicate items that + * are supposed to end up on the new right sibling page following the + * anticipated page split. A future deduplication pass of future right + * sibling page might take care of it. (This is why the first single value + * strategy deduplication pass for a given leaf page will generally find only + * plain non-pivot tuples -- see _bt_do_singleval() comments.) + */ +static void +_bt_singleval_fillfactor(Page page, BTDedupState state, Size newitemsz) +{ + Size leftfree; + int reduction; + + /* This calculation needs to match nbtsplitloc.c */ + leftfree = PageGetPageSize(page) - SizeOfPageHeaderData - + MAXALIGN(sizeof(BTPageOpaqueData)); + /* Subtract size of new high key (includes pivot heap TID space) */ + leftfree -= newitemsz + MAXALIGN(sizeof(ItemPointerData)); + + /* + * Reduce maxpostingsize by an amount equal to target free space on left + * half of page + */ + reduction = leftfree * ((100 - BTREE_SINGLEVAL_FILLFACTOR) / 100.0); + if (state->maxpostingsize > reduction) + state->maxpostingsize -= reduction; + else + state->maxpostingsize = 0; +} + +/* + * Build a posting list tuple based on caller's "base" index tuple and list of + * heap TIDs. When nhtids == 1, builds a standard non-pivot tuple without a + * posting list. (Posting list tuples can never have a single heap TID, partly + * because that ensures that deduplication always reduces final MAXALIGN()'d + * size of entire tuple.) + * + * Convention is that posting list starts at a MAXALIGN()'d offset (rather + * than a SHORTALIGN()'d offset), in line with the approach taken when + * appending a heap TID to new pivot tuple/high key during suffix truncation. + * This sometimes wastes a little space that was only needed as alignment + * padding in the original tuple. Following this convention simplifies the + * space accounting used when deduplicating a page (the same convention + * simplifies the accounting for choosing a point to split a page at). + * + * Note: Caller's "htids" array must be unique and already in ascending TID + * order. Any existing heap TIDs from "base" won't automatically appear in + * returned posting list tuple (they must be included in htids array.) + */ +IndexTuple +_bt_form_posting(IndexTuple base, ItemPointer htids, int nhtids) +{ + uint32 keysize, + newsize; + IndexTuple itup; + + if (BTreeTupleIsPosting(base)) + keysize = BTreeTupleGetPostingOffset(base); + else + keysize = IndexTupleSize(base); + + Assert(!BTreeTupleIsPivot(base)); + Assert(nhtids > 0 && nhtids <= PG_UINT16_MAX); + Assert(keysize == MAXALIGN(keysize)); + + /* Determine final size of new tuple */ + if (nhtids > 1) + newsize = MAXALIGN(keysize + + nhtids * sizeof(ItemPointerData)); + else + newsize = keysize; + + Assert(newsize <= INDEX_SIZE_MASK); + Assert(newsize == MAXALIGN(newsize)); + + /* Allocate memory using palloc0() (matches index_form_tuple()) */ + itup = palloc0(newsize); + memcpy(itup, base, keysize); + itup->t_info &= ~INDEX_SIZE_MASK; + itup->t_info |= newsize; + if (nhtids > 1) + { + /* Form posting list tuple */ + BTreeTupleSetPosting(itup, nhtids, keysize); + memcpy(BTreeTupleGetPosting(itup), htids, + sizeof(ItemPointerData) * nhtids); + Assert(_bt_posting_valid(itup)); + } + else + { + /* Form standard non-pivot tuple */ + itup->t_info &= ~INDEX_ALT_TID_MASK; + ItemPointerCopy(htids, &itup->t_tid); + Assert(ItemPointerIsValid(&itup->t_tid)); + } + + return itup; +} + +/* + * Generate a replacement tuple by "updating" a posting list tuple so that it + * no longer has TIDs that need to be deleted. + * + * Used by VACUUM. Caller's vacposting argument points to the existing + * posting list tuple to be updated. + * + * On return, caller's vacposting argument will point to final "updated" + * tuple, which will be palloc()'d in caller's memory context. + */ +void +_bt_update_posting(BTVacuumPosting vacposting) +{ + IndexTuple origtuple = vacposting->itup; + uint32 keysize, + newsize; + IndexTuple itup; + int nhtids; + int ui, + d; + ItemPointer htids; + + nhtids = BTreeTupleGetNPosting(origtuple) - vacposting->ndeletedtids; + + Assert(_bt_posting_valid(origtuple)); + Assert(nhtids > 0 && nhtids < BTreeTupleGetNPosting(origtuple)); + + /* + * Determine final size of new tuple. + * + * This calculation needs to match the code used within _bt_form_posting() + * for new posting list tuples. We avoid calling _bt_form_posting() here + * to save ourselves a second memory allocation for a htids workspace. + */ + keysize = BTreeTupleGetPostingOffset(origtuple); + if (nhtids > 1) + newsize = MAXALIGN(keysize + + nhtids * sizeof(ItemPointerData)); + else + newsize = keysize; + + Assert(newsize <= INDEX_SIZE_MASK); + Assert(newsize == MAXALIGN(newsize)); + + /* Allocate memory using palloc0() (matches index_form_tuple()) */ + itup = palloc0(newsize); + memcpy(itup, origtuple, keysize); + itup->t_info &= ~INDEX_SIZE_MASK; + itup->t_info |= newsize; + + if (nhtids > 1) + { + /* Form posting list tuple */ + BTreeTupleSetPosting(itup, nhtids, keysize); + htids = BTreeTupleGetPosting(itup); + } + else + { + /* Form standard non-pivot tuple */ + itup->t_info &= ~INDEX_ALT_TID_MASK; + htids = &itup->t_tid; + } + + ui = 0; + d = 0; + for (int i = 0; i < BTreeTupleGetNPosting(origtuple); i++) + { + if (d < vacposting->ndeletedtids && vacposting->deletetids[d] == i) + { + d++; + continue; + } + htids[ui++] = *BTreeTupleGetPostingN(origtuple, i); + } + Assert(ui == nhtids); + Assert(d == vacposting->ndeletedtids); + Assert(nhtids == 1 || _bt_posting_valid(itup)); + Assert(nhtids > 1 || ItemPointerIsValid(&itup->t_tid)); + + /* vacposting arg's itup will now point to updated version */ + vacposting->itup = itup; +} + +/* + * Prepare for a posting list split by swapping heap TID in newitem with heap + * TID from original posting list (the 'oposting' heap TID located at offset + * 'postingoff'). Modifies newitem, so caller should pass their own private + * copy that can safely be modified. + * + * Returns new posting list tuple, which is palloc()'d in caller's context. + * This is guaranteed to be the same size as 'oposting'. Modified newitem is + * what caller actually inserts. (This happens inside the same critical + * section that performs an in-place update of old posting list using new + * posting list returned here.) + * + * While the keys from newitem and oposting must be opclass equal, and must + * generate identical output when run through the underlying type's output + * function, it doesn't follow that their representations match exactly. + * Caller must avoid assuming that there can't be representational differences + * that make datums from oposting bigger or smaller than the corresponding + * datums from newitem. For example, differences in TOAST input state might + * break a faulty assumption about tuple size (the executor is entitled to + * apply TOAST compression based on its own criteria). It also seems possible + * that further representational variation will be introduced in the future, + * in order to support nbtree features like page-level prefix compression. + * + * See nbtree/README for details on the design of posting list splits. + */ +IndexTuple +_bt_swap_posting(IndexTuple newitem, IndexTuple oposting, int postingoff) +{ + int nhtids; + char *replacepos; + char *replaceposright; + Size nmovebytes; + IndexTuple nposting; + + nhtids = BTreeTupleGetNPosting(oposting); + Assert(_bt_posting_valid(oposting)); + + /* + * The postingoff argument originated as a _bt_binsrch_posting() return + * value. It will be 0 in the event of corruption that makes a leaf page + * contain a non-pivot tuple that's somehow identical to newitem (no two + * non-pivot tuples should ever have the same TID). This has been known + * to happen in the field from time to time. + * + * Perform a basic sanity check to catch this case now. + */ + if (!(postingoff > 0 && postingoff < nhtids)) + elog(ERROR, "posting list tuple with %d items cannot be split at offset %d", + nhtids, postingoff); + + /* + * Move item pointers in posting list to make a gap for the new item's + * heap TID. We shift TIDs one place to the right, losing original + * rightmost TID. (nmovebytes must not include TIDs to the left of + * postingoff, nor the existing rightmost/max TID that gets overwritten.) + */ + nposting = CopyIndexTuple(oposting); + replacepos = (char *) BTreeTupleGetPostingN(nposting, postingoff); + replaceposright = (char *) BTreeTupleGetPostingN(nposting, postingoff + 1); + nmovebytes = (nhtids - postingoff - 1) * sizeof(ItemPointerData); + memmove(replaceposright, replacepos, nmovebytes); + + /* Fill the gap at postingoff with TID of new item (original new TID) */ + Assert(!BTreeTupleIsPivot(newitem) && !BTreeTupleIsPosting(newitem)); + ItemPointerCopy(&newitem->t_tid, (ItemPointer) replacepos); + + /* Now copy oposting's rightmost/max TID into new item (final new TID) */ + ItemPointerCopy(BTreeTupleGetMaxHeapTID(oposting), &newitem->t_tid); + + Assert(ItemPointerCompare(BTreeTupleGetMaxHeapTID(nposting), + BTreeTupleGetHeapTID(newitem)) < 0); + Assert(_bt_posting_valid(nposting)); + + return nposting; +} + +/* + * Verify posting list invariants for "posting", which must be a posting list + * tuple. Used within assertions. + */ +#ifdef USE_ASSERT_CHECKING +static bool +_bt_posting_valid(IndexTuple posting) +{ + ItemPointerData last; + ItemPointer htid; + + if (!BTreeTupleIsPosting(posting) || BTreeTupleGetNPosting(posting) < 2) + return false; + + /* Remember first heap TID for loop */ + ItemPointerCopy(BTreeTupleGetHeapTID(posting), &last); + if (!ItemPointerIsValid(&last)) + return false; + + /* Iterate, starting from second TID */ + for (int i = 1; i < BTreeTupleGetNPosting(posting); i++) + { + htid = BTreeTupleGetPostingN(posting, i); + + if (!ItemPointerIsValid(htid)) + return false; + if (ItemPointerCompare(htid, &last) <= 0) + return false; + ItemPointerCopy(htid, &last); + } + + return true; +} +#endif |