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/*-
* Copyright (c) 2014-present MongoDB, Inc.
* Copyright (c) 2008-2014 WiredTiger, Inc.
* All rights reserved.
*
* See the file LICENSE for redistribution information.
*/
#include "wt_internal.h"
static void __inmem_col_fix(WT_SESSION_IMPL *, WT_PAGE *);
static void __inmem_col_int(WT_SESSION_IMPL *, WT_PAGE *);
static int __inmem_col_var(WT_SESSION_IMPL *, WT_PAGE *, uint64_t, bool *, size_t *);
static int __inmem_row_int(WT_SESSION_IMPL *, WT_PAGE *, size_t *);
static int __inmem_row_leaf(WT_SESSION_IMPL *, WT_PAGE *, bool *);
static int __inmem_row_leaf_entries(WT_SESSION_IMPL *, const WT_PAGE_HEADER *, uint32_t *);
/*
* __wt_page_alloc --
* Create or read a page into the cache.
*/
int
__wt_page_alloc(
WT_SESSION_IMPL *session, uint8_t type, uint32_t alloc_entries, bool alloc_refs, WT_PAGE **pagep)
{
WT_CACHE *cache;
WT_DECL_RET;
WT_PAGE *page;
WT_PAGE_INDEX *pindex;
size_t size;
uint32_t i;
void *p;
*pagep = NULL;
cache = S2C(session)->cache;
page = NULL;
size = sizeof(WT_PAGE);
switch (type) {
case WT_PAGE_COL_FIX:
case WT_PAGE_COL_INT:
case WT_PAGE_ROW_INT:
break;
case WT_PAGE_COL_VAR:
/*
* Variable-length column-store leaf page: allocate memory to describe the page's contents
* with the initial allocation.
*/
size += alloc_entries * sizeof(WT_COL);
break;
case WT_PAGE_ROW_LEAF:
/*
* Row-store leaf page: allocate memory to describe the page's contents with the initial
* allocation.
*/
size += alloc_entries * sizeof(WT_ROW);
break;
default:
return (__wt_illegal_value(session, type));
}
WT_RET(__wt_calloc(session, 1, size, &page));
page->type = type;
page->read_gen = WT_READGEN_NOTSET;
switch (type) {
case WT_PAGE_COL_FIX:
page->entries = alloc_entries;
break;
case WT_PAGE_COL_INT:
case WT_PAGE_ROW_INT:
WT_ASSERT(session, alloc_entries != 0);
/*
* Internal pages have an array of references to objects so they can split. Allocate the
* array of references and optionally, the objects to which they point.
*/
WT_ERR(
__wt_calloc(session, 1, sizeof(WT_PAGE_INDEX) + alloc_entries * sizeof(WT_REF *), &p));
size += sizeof(WT_PAGE_INDEX) + alloc_entries * sizeof(WT_REF *);
pindex = p;
pindex->index = (WT_REF **)((WT_PAGE_INDEX *)p + 1);
pindex->entries = alloc_entries;
WT_INTL_INDEX_SET(page, pindex);
if (alloc_refs)
for (i = 0; i < pindex->entries; ++i) {
WT_ERR(__wt_calloc_one(session, &pindex->index[i]));
size += sizeof(WT_REF);
}
if (0) {
err:
if ((pindex = WT_INTL_INDEX_GET_SAFE(page)) != NULL) {
for (i = 0; i < pindex->entries; ++i)
__wt_free(session, pindex->index[i]);
__wt_free(session, pindex);
}
__wt_free(session, page);
return (ret);
}
break;
case WT_PAGE_COL_VAR:
page->pg_var = alloc_entries == 0 ? NULL : (WT_COL *)((uint8_t *)page + sizeof(WT_PAGE));
page->entries = alloc_entries;
break;
case WT_PAGE_ROW_LEAF:
page->pg_row = alloc_entries == 0 ? NULL : (WT_ROW *)((uint8_t *)page + sizeof(WT_PAGE));
page->entries = alloc_entries;
break;
default:
return (__wt_illegal_value(session, type));
}
/* Increment the cache statistics. */
__wt_cache_page_inmem_incr(session, page, size);
(void)__wt_atomic_add64(&cache->pages_inmem, 1);
page->cache_create_gen = cache->evict_pass_gen;
*pagep = page;
return (0);
}
/*
* __page_inmem_prepare_update --
* Create the actual update for a prepared value.
*/
static int
__page_inmem_prepare_update(WT_SESSION_IMPL *session, WT_ITEM *value, WT_CELL_UNPACK_KV *unpack,
WT_UPDATE **updp, size_t *sizep)
{
WT_DECL_RET;
WT_UPDATE *upd, *tombstone;
size_t size, total_size;
*sizep = 0;
tombstone = upd = NULL;
total_size = 0;
WT_RET(__wt_upd_alloc(session, value, WT_UPDATE_STANDARD, &upd, &size));
total_size += size;
upd->durable_ts = unpack->tw.durable_start_ts;
upd->start_ts = unpack->tw.start_ts;
upd->txnid = unpack->tw.start_txn;
F_SET(upd, WT_UPDATE_PREPARE_RESTORED_FROM_DS);
/*
* Instantiate both update and tombstone if the prepared update is a tombstone. This is required
* to ensure that written prepared delete operation must be removed from the data store, when
* the prepared transaction gets rollback.
*/
if (WT_TIME_WINDOW_HAS_STOP(&unpack->tw)) {
WT_ERR(__wt_upd_alloc_tombstone(session, &tombstone, &size));
total_size += size;
tombstone->durable_ts = WT_TS_NONE;
tombstone->start_ts = unpack->tw.stop_ts;
tombstone->txnid = unpack->tw.stop_txn;
tombstone->prepare_state = WT_PREPARE_INPROGRESS;
F_SET(tombstone, WT_UPDATE_PREPARE_RESTORED_FROM_DS);
/*
* Mark the update also as in-progress if the update and tombstone are from same transaction
* by comparing both the transaction and timestamps as the transaction information gets lost
* after restart.
*/
if (unpack->tw.start_ts == unpack->tw.stop_ts &&
unpack->tw.durable_start_ts == unpack->tw.durable_stop_ts &&
unpack->tw.start_txn == unpack->tw.stop_txn) {
upd->durable_ts = WT_TS_NONE;
upd->prepare_state = WT_PREPARE_INPROGRESS;
}
tombstone->next = upd;
*updp = tombstone;
} else {
upd->durable_ts = WT_TS_NONE;
upd->prepare_state = WT_PREPARE_INPROGRESS;
*updp = upd;
}
*sizep = total_size;
return (0);
err:
__wt_free(session, upd);
__wt_free(session, tombstone);
return (ret);
}
/*
* __wt_page_inmem_prepare --
* Instantiate prepared updates.
*/
int
__wt_page_inmem_prepare(WT_SESSION_IMPL *session, WT_REF *ref)
{
WT_CELL *cell;
WT_CELL_UNPACK_KV unpack;
WT_COL *cip;
WT_CURSOR_BTREE cbt;
WT_DECL_ITEM(key);
WT_DECL_ITEM(value);
WT_DECL_RET;
WT_PAGE *page;
WT_ROW *rip;
WT_UPDATE *upd;
size_t size, total_size;
uint64_t recno, rle;
uint32_t i;
page = ref->page;
upd = NULL;
total_size = 0;
/*
* We don't handle in-memory prepare resolution here, and prepare only applies to row-store and
* variable-length column store leaf pages.
*/
WT_ASSERT(session, !F_ISSET(S2C(session), WT_CONN_IN_MEMORY));
WT_ASSERT(session, page->type == WT_PAGE_COL_VAR || page->type == WT_PAGE_ROW_LEAF);
__wt_btcur_init(session, &cbt);
__wt_btcur_open(&cbt);
WT_ERR(__wt_scr_alloc(session, 0, &value));
if (page->type == WT_PAGE_COL_VAR) {
recno = ref->ref_recno;
WT_COL_FOREACH (page, cip, i) {
/* Search for prepare records. */
cell = WT_COL_PTR(page, cip);
__wt_cell_unpack_kv(session, page->dsk, cell, &unpack);
rle = __wt_cell_rle(&unpack);
if (!unpack.tw.prepare) {
recno += rle;
continue;
}
/* Get the value. */
WT_ERR(__wt_page_cell_data_ref(session, page, &unpack, value));
/* For each record, create an update to resolve the prepare. */
for (; rle > 0; --rle, ++recno) {
/* Create an update to resolve the prepare. */
WT_ERR(__page_inmem_prepare_update(session, value, &unpack, &upd, &size));
total_size += size;
/* Search the page and apply the modification. */
WT_ERR(__wt_col_search(&cbt, recno, ref, true, NULL));
#ifdef HAVE_DIAGNOSTIC
WT_ERR(__wt_col_modify(&cbt, recno, NULL, upd, WT_UPDATE_INVALID, true, true));
#else
WT_ERR(__wt_col_modify(&cbt, recno, NULL, upd, WT_UPDATE_INVALID, true));
#endif
upd = NULL;
}
}
} else {
WT_ERR(__wt_scr_alloc(session, 0, &key));
WT_ROW_FOREACH (page, rip, i) {
/* Search for prepare records. */
__wt_row_leaf_value_cell(session, page, rip, &unpack);
if (!unpack.tw.prepare)
continue;
/* Get the key/value pair and create an update to resolve the prepare. */
WT_ERR(__wt_row_leaf_key(session, page, rip, key, false));
WT_ERR(__wt_page_cell_data_ref(session, page, &unpack, value));
WT_ERR(__page_inmem_prepare_update(session, value, &unpack, &upd, &size));
total_size += size;
/* Search the page and apply the modification. */
WT_ERR(__wt_row_search(&cbt, key, true, ref, true, NULL));
#ifdef HAVE_DIAGNOSTIC
WT_ERR(__wt_row_modify(&cbt, key, NULL, upd, WT_UPDATE_INVALID, true, true));
#else
WT_ERR(__wt_row_modify(&cbt, key, NULL, upd, WT_UPDATE_INVALID, true));
#endif
upd = NULL;
}
}
__wt_cache_page_inmem_incr(session, page, total_size);
if (0) {
err:
__wt_free_update_list(session, &upd);
}
WT_TRET(__wt_btcur_close(&cbt, true));
__wt_scr_free(session, &key);
__wt_scr_free(session, &value);
return (ret);
}
/*
* __wt_page_inmem --
* Build in-memory page information.
*/
int
__wt_page_inmem(WT_SESSION_IMPL *session, WT_REF *ref, const void *image, uint32_t flags,
WT_PAGE **pagep, bool *preparedp)
{
WT_DECL_RET;
WT_PAGE *page;
const WT_PAGE_HEADER *dsk;
size_t size;
uint32_t alloc_entries;
*pagep = NULL;
if (preparedp != NULL)
*preparedp = false;
dsk = image;
alloc_entries = 0;
/*
* Figure out how many underlying objects the page references so we can allocate them along with
* the page.
*/
switch (dsk->type) {
case WT_PAGE_COL_FIX:
case WT_PAGE_COL_INT:
case WT_PAGE_COL_VAR:
/*
* Column-store leaf page entries map one-to-one to the number of physical entries on the
* page (each physical entry is a value item). Note this value isn't necessarily correct, we
* may skip values when reading the disk image.
*
* Column-store internal page entries map one-to-one to the number of physical entries on
* the page (each entry is a location cookie).
*/
alloc_entries = dsk->u.entries;
break;
case WT_PAGE_ROW_INT:
/*
* Row-store internal page entries map one-to-two to the number of physical entries on the
* page (each entry is a key and location cookie pair).
*/
alloc_entries = dsk->u.entries / 2;
break;
case WT_PAGE_ROW_LEAF:
/*
* If the "no empty values" flag is set, row-store leaf page entries map one-to-one to the
* number of physical entries on the page (each physical entry is a key or value item). If
* that flag is not set, there are more keys than values, we have to walk the page to figure
* it out. Note this value isn't necessarily correct, we may skip values when reading the
* disk image.
*/
if (F_ISSET(dsk, WT_PAGE_EMPTY_V_ALL))
alloc_entries = dsk->u.entries;
else if (F_ISSET(dsk, WT_PAGE_EMPTY_V_NONE))
alloc_entries = dsk->u.entries / 2;
else
WT_RET(__inmem_row_leaf_entries(session, dsk, &alloc_entries));
break;
default:
return (__wt_illegal_value(session, dsk->type));
}
/* Allocate and initialize a new WT_PAGE. */
WT_RET(__wt_page_alloc(session, dsk->type, alloc_entries, true, &page));
page->dsk = dsk;
F_SET_ATOMIC_16(page, flags);
/*
* Track the memory allocated to build this page so we can update the cache statistics in a
* single call. If the disk image is in allocated memory, start with that.
*
* Accounting is based on the page-header's in-memory disk size instead of the buffer memory
* used to instantiate the page image even though the values might not match exactly, because
* that's the only value we have when discarding the page image and accounting needs to match.
*/
size = LF_ISSET(WT_PAGE_DISK_ALLOC) ? dsk->mem_size : 0;
switch (page->type) {
case WT_PAGE_COL_FIX:
__inmem_col_fix(session, page);
break;
case WT_PAGE_COL_INT:
__inmem_col_int(session, page);
break;
case WT_PAGE_COL_VAR:
WT_ERR(__inmem_col_var(session, page, dsk->recno, preparedp, &size));
break;
case WT_PAGE_ROW_INT:
WT_ERR(__inmem_row_int(session, page, &size));
break;
case WT_PAGE_ROW_LEAF:
WT_ERR(__inmem_row_leaf(session, page, preparedp));
break;
default:
WT_ERR(__wt_illegal_value(session, page->type));
}
/* Update the page's cache statistics. */
__wt_cache_page_inmem_incr(session, page, size);
if (LF_ISSET(WT_PAGE_DISK_ALLOC))
__wt_cache_page_image_incr(session, page);
/* Link the new internal page to the parent. */
if (ref != NULL) {
switch (page->type) {
case WT_PAGE_COL_INT:
case WT_PAGE_ROW_INT:
page->pg_intl_parent_ref = ref;
break;
}
ref->page = page;
}
*pagep = page;
return (0);
err:
__wt_page_out(session, &page);
return (ret);
}
/*
* __inmem_col_fix --
* Build in-memory index for fixed-length column-store leaf pages.
*/
static void
__inmem_col_fix(WT_SESSION_IMPL *session, WT_PAGE *page)
{
WT_BTREE *btree;
const WT_PAGE_HEADER *dsk;
btree = S2BT(session);
dsk = page->dsk;
page->pg_fix_bitf = WT_PAGE_HEADER_BYTE(btree, dsk);
}
/*
* __inmem_col_int --
* Build in-memory index for column-store internal pages.
*/
static void
__inmem_col_int(WT_SESSION_IMPL *session, WT_PAGE *page)
{
WT_CELL_UNPACK_ADDR unpack;
WT_PAGE_INDEX *pindex;
WT_REF **refp, *ref;
uint32_t hint;
/*
* Walk the page, building references: the page contains value items. The value items are
* on-page items (WT_CELL_VALUE).
*/
pindex = WT_INTL_INDEX_GET_SAFE(page);
refp = pindex->index;
hint = 0;
WT_CELL_FOREACH_ADDR (session, page->dsk, unpack) {
ref = *refp++;
ref->home = page;
ref->pindex_hint = hint++;
ref->addr = unpack.cell;
ref->ref_recno = unpack.v;
F_SET(ref, unpack.type == WT_CELL_ADDR_INT ? WT_REF_FLAG_INTERNAL : WT_REF_FLAG_LEAF);
}
WT_CELL_FOREACH_END;
}
/*
* __inmem_col_var_repeats --
* Count the number of repeat entries on the page.
*/
static void
__inmem_col_var_repeats(WT_SESSION_IMPL *session, WT_PAGE *page, uint32_t *np)
{
WT_CELL_UNPACK_KV unpack;
*np = 0;
/* Walk the page, counting entries for the repeats array. */
WT_CELL_FOREACH_KV (session, page->dsk, unpack) {
if (__wt_cell_rle(&unpack) > 1)
++*np;
}
WT_CELL_FOREACH_END;
}
/*
* __inmem_col_var --
* Build in-memory index for variable-length, data-only leaf pages in column-store trees.
*/
static int
__inmem_col_var(
WT_SESSION_IMPL *session, WT_PAGE *page, uint64_t recno, bool *preparedp, size_t *sizep)
{
WT_CELL_UNPACK_KV unpack;
WT_COL *cip;
WT_COL_RLE *repeats;
size_t size;
uint64_t rle;
uint32_t indx, n, repeat_off;
bool prepare;
void *p;
repeats = NULL;
repeat_off = 0;
prepare = false;
/*
* Walk the page, building references: the page contains unsorted value items. The value items
* are on-page (WT_CELL_VALUE), overflow items (WT_CELL_VALUE_OVFL) or deleted items
* (WT_CELL_DEL).
*/
indx = 0;
cip = page->pg_var;
WT_CELL_FOREACH_KV (session, page->dsk, unpack) {
WT_COL_PTR_SET(cip, WT_PAGE_DISK_OFFSET(page, unpack.cell));
cip++;
/*
* Add records with repeat counts greater than 1 to an array we use for fast lookups. The
* first entry we find needing the repeats array triggers a re-walk from the start of the
* page to determine the size of the array.
*/
rle = __wt_cell_rle(&unpack);
if (rle > 1) {
if (repeats == NULL) {
__inmem_col_var_repeats(session, page, &n);
size = sizeof(WT_COL_VAR_REPEAT) + (n + 1) * sizeof(WT_COL_RLE);
WT_RET(__wt_calloc(session, 1, size, &p));
*sizep += size;
page->u.col_var.repeats = p;
page->pg_var_nrepeats = n;
repeats = page->pg_var_repeats;
}
repeats[repeat_off].indx = indx;
repeats[repeat_off].recno = recno;
repeats[repeat_off++].rle = rle;
}
/* If we find a prepare, we'll have to instantiate it in the update chain later. */
if (unpack.tw.prepare)
prepare = true;
indx++;
recno += rle;
}
WT_CELL_FOREACH_END;
if (preparedp != NULL && prepare)
*preparedp = true;
return (0);
}
/*
* __inmem_row_int --
* Build in-memory index for row-store internal pages.
*/
static int
__inmem_row_int(WT_SESSION_IMPL *session, WT_PAGE *page, size_t *sizep)
{
WT_BTREE *btree;
WT_CELL_UNPACK_ADDR unpack;
WT_DECL_ITEM(current);
WT_DECL_RET;
WT_PAGE_INDEX *pindex;
WT_REF *ref, **refp;
uint32_t hint;
bool overflow_keys;
btree = S2BT(session);
WT_RET(__wt_scr_alloc(session, 0, ¤t));
/*
* Walk the page, instantiating keys: the page contains sorted key and location cookie pairs.
* Keys are on-page/overflow items and location cookies are WT_CELL_ADDR_XXX items.
*/
pindex = WT_INTL_INDEX_GET_SAFE(page);
refp = pindex->index;
overflow_keys = false;
hint = 0;
WT_CELL_FOREACH_ADDR (session, page->dsk, unpack) {
ref = *refp;
ref->home = page;
ref->pindex_hint = hint++;
switch (unpack.type) {
case WT_CELL_ADDR_INT:
F_SET(ref, WT_REF_FLAG_INTERNAL);
break;
case WT_CELL_ADDR_DEL:
case WT_CELL_ADDR_LEAF:
case WT_CELL_ADDR_LEAF_NO:
F_SET(ref, WT_REF_FLAG_LEAF);
break;
}
switch (unpack.type) {
case WT_CELL_KEY:
/*
* Note: we don't Huffman encode internal page keys, there's no decoding work to do.
*/
__wt_ref_key_onpage_set(page, ref, &unpack);
break;
case WT_CELL_KEY_OVFL:
/*
* Instantiate any overflow keys; WiredTiger depends on this, assuming any overflow key
* is instantiated, and any keys that aren't instantiated cannot be overflow items.
*/
WT_ERR(__wt_dsk_cell_data_ref(session, page->type, &unpack, current));
WT_ERR(__wt_row_ikey_incr(session, page, WT_PAGE_DISK_OFFSET(page, unpack.cell),
current->data, current->size, ref));
*sizep += sizeof(WT_IKEY) + current->size;
overflow_keys = true;
break;
case WT_CELL_ADDR_DEL:
/*
* A cell may reference a deleted leaf page: if a leaf page was deleted without being
* read (fast truncate), and the deletion committed, but older transactions in the
* system required the previous version of the page to remain available, a special
* deleted-address type cell is written. We'll see that cell on a page if we read from a
* checkpoint including a deleted cell or if we crash/recover and start off from such a
* checkpoint (absent running recovery, a version of the page without the deleted cell
* would eventually have been written). If we crash and recover to a page with a
* deleted-address cell, we want to discard the page from the backing store (it was
* never discarded), and, of course, by definition no earlier transaction will ever need
* it.
*
* Re-create the state of a deleted page.
*/
ref->addr = unpack.cell;
WT_REF_SET_STATE(ref, WT_REF_DELETED);
++refp;
/*
* If the tree is already dirty and so will be written, mark the page dirty. (We want to
* free the deleted pages, but if the handle is read-only or if the application never
* modifies the tree, we're not able to do so.)
*/
if (btree->modified) {
WT_ERR(__wt_page_modify_init(session, page));
__wt_page_modify_set(session, page);
}
break;
case WT_CELL_ADDR_INT:
case WT_CELL_ADDR_LEAF:
case WT_CELL_ADDR_LEAF_NO:
ref->addr = unpack.cell;
++refp;
break;
default:
WT_ERR(__wt_illegal_value(session, unpack.type));
}
}
WT_CELL_FOREACH_END;
/*
* We track if an internal page has backing overflow keys, as overflow keys limit the eviction
* we can do during a checkpoint. (This is only for historical tables, reconciliation no longer
* writes overflow cookies on internal pages, no matter the size of the key.)
*/
if (overflow_keys)
F_SET_ATOMIC_16(page, WT_PAGE_INTL_OVERFLOW_KEYS);
err:
__wt_scr_free(session, ¤t);
return (ret);
}
/*
* __inmem_row_leaf_entries --
* Return the number of entries for row-store leaf pages.
*/
static int
__inmem_row_leaf_entries(WT_SESSION_IMPL *session, const WT_PAGE_HEADER *dsk, uint32_t *nindxp)
{
WT_CELL_UNPACK_KV unpack;
uint32_t nindx;
/*
* Leaf row-store page entries map to a maximum of one-to-one to the number of physical entries
* on the page (each physical entry might be a key without a subsequent data item). To avoid
* over-allocation in workloads without empty data items, first walk the page counting the
* number of keys, then allocate the indices.
*
* The page contains key/data pairs. Keys are on-page (WT_CELL_KEY) or overflow
* (WT_CELL_KEY_OVFL) items, data are either non-existent or a single on-page (WT_CELL_VALUE) or
* overflow (WT_CELL_VALUE_OVFL) item.
*/
nindx = 0;
WT_CELL_FOREACH_KV (session, dsk, unpack) {
switch (unpack.type) {
case WT_CELL_KEY:
case WT_CELL_KEY_OVFL:
++nindx;
break;
case WT_CELL_VALUE:
case WT_CELL_VALUE_OVFL:
break;
default:
return (__wt_illegal_value(session, unpack.type));
}
}
WT_CELL_FOREACH_END;
*nindxp = nindx;
return (0);
}
/*
* __inmem_row_leaf --
* Build in-memory index for row-store leaf pages.
*/
static int
__inmem_row_leaf(WT_SESSION_IMPL *session, WT_PAGE *page, bool *preparedp)
{
WT_BTREE *btree;
WT_CELL_UNPACK_KV unpack;
WT_DECL_RET;
WT_ROW *rip;
uint32_t best_prefix_count, best_prefix_start, best_prefix_stop;
uint32_t last_slot, prefix_count, prefix_start, prefix_stop, slot;
uint8_t smallest_prefix;
bool prepare;
btree = S2BT(session);
last_slot = 0;
prepare = false;
/* The code depends on the prefix count variables, other initialization shouldn't matter. */
best_prefix_count = prefix_count = 0;
smallest_prefix = 0; /* [-Wconditional-uninitialized] */
prefix_start = prefix_stop = 0; /* [-Wconditional-uninitialized] */
best_prefix_start = best_prefix_stop = 0; /* [-Wconditional-uninitialized] */
/* Walk the page, building indices. */
rip = page->pg_row;
WT_CELL_FOREACH_KV (session, page->dsk, unpack) {
switch (unpack.type) {
case WT_CELL_KEY:
/*
* Simple keys and prefix-compressed keys can be directly referenced on the page to
* avoid repeatedly unpacking their cells.
*
* Review groups of prefix-compressed keys, and track the biggest group as the page's
* prefix. What we're finding is the biggest group of prefix-compressed keys we can
* immediately build using a previous key plus their suffix bytes, without rolling
* forward through intermediate keys. We save that information on the page and then
* never physically instantiate those keys, avoiding memory amplification for pages with
* a page-wide prefix. On the first of a group of prefix-compressed keys, track the slot
* of the fully-instantiated key from which it's derived and the current key's prefix
* length. On subsequent keys, if the key can be built from the original key plus the
* current key's suffix bytes, update the maximum slot to which the prefix applies and
* the smallest prefix length.
*
* Groups of prefix-compressed keys end when a key is not prefix-compressed (ignoring
* overflow keys), or the key's prefix length increases. A prefix length decreasing is
* OK, it only means fewer bytes taken from the original key. A prefix length increasing
* doesn't necessarily end a group of prefix-compressed keys as we might be able to
* build a subsequent key using the original key and the key's suffix bytes, that is the
* prefix length could increase and then decrease to the same prefix length as before
* and those latter keys could be built without rolling forward through intermediate
* keys.
*
* However, that gets tricky: once a key prefix grows, we can never include a prefix
* smaller than the smallest prefix found so far, in the group, as a subsequent key
* prefix larger than the smallest prefix found so far might include bytes not present
* in the original instantiated key. Growing and shrinking is complicated to track, so
* rather than code up that complexity, we close out a group whenever the prefix grows.
* Plus, growing has additional issues. Any key with a larger prefix cannot be
* instantiated without rolling forward through intermediate keys, and so while such a
* key isn't required to close out the prefix group in all cases, it's not a useful
* entry for finding the best group of prefix-compressed keys, either, it's only
* possible keys after the prefix shrinks again that are potentially worth including in
* a group.
*/
slot = WT_ROW_SLOT(page, rip);
if (unpack.prefix == 0) {
/* If the last prefix group was the best, track it. */
if (prefix_count > best_prefix_count) {
best_prefix_start = prefix_start;
best_prefix_stop = prefix_stop;
best_prefix_count = prefix_count;
}
prefix_count = 0;
prefix_start = slot;
} else {
/* Check for starting or continuing a prefix group. */
if (prefix_count == 0 ||
(last_slot == slot - 1 && unpack.prefix <= smallest_prefix)) {
smallest_prefix = unpack.prefix;
last_slot = prefix_stop = slot;
++prefix_count;
}
}
__wt_row_leaf_key_set(page, rip, &unpack);
++rip;
continue;
case WT_CELL_KEY_OVFL:
/*
* Prefix compression skips overflow items, ignore this slot. The last slot value is
* only used inside a group of prefix-compressed keys, so blindly increment it, it's not
* used unless the count of prefix-compressed keys is non-zero.
*/
++last_slot;
__wt_row_leaf_key_set(page, rip, &unpack);
++rip;
continue;
case WT_CELL_VALUE:
/*
* Simple values without compression can be directly referenced on the page to avoid
* repeatedly unpacking their cells.
*
* The visibility information is not referenced on the page so we need to ensure that
* the value is globally visible at the point in time where we read the page into cache.
*/
if (!btree->huffman_value &&
(WT_TIME_WINDOW_IS_EMPTY(&unpack.tw) ||
(!WT_TIME_WINDOW_HAS_STOP(&unpack.tw) &&
__wt_txn_tw_start_visible_all(session, &unpack.tw))))
__wt_row_leaf_value_set(rip - 1, &unpack);
break;
case WT_CELL_VALUE_OVFL:
break;
default:
WT_ERR(__wt_illegal_value(session, unpack.type));
}
/* If we find a prepare, we'll have to instantiate it in the update chain later. */
if (unpack.tw.prepare)
prepare = true;
}
WT_CELL_FOREACH_END;
/* If the last prefix group was the best, track it. Save the best prefix group for the page. */
if (prefix_count > best_prefix_count) {
best_prefix_start = prefix_start;
best_prefix_stop = prefix_stop;
}
page->prefix_start = best_prefix_start;
page->prefix_stop = best_prefix_stop;
/*
* Backward cursor traversal can be too slow if we're forced to process long stretches of
* prefix-compressed keys to create every key as we walk backwards through the page, and we
* handle that by instantiating periodic keys when backward cursor traversal enters a new page.
* Mark the page as not needing that work if there aren't stretches of prefix-compressed keys.
*/
if (best_prefix_count <= 10)
F_SET_ATOMIC_16(page, WT_PAGE_BUILD_KEYS);
if (preparedp != NULL && prepare)
*preparedp = true;
err:
return (ret);
}
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