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Check for invalid down link while prefetching B-Tree leave pages for index-only scan #537

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Merged
knizhnik merged 5 commits into from
Dec 13, 2024
Merged

Check for invalid down link while prefetching B-Tree leave pages for index-only scan #537

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7c1cbcb
Check for invalid down link while prefetching B-Tree leave pages for …
knizhnik Nov 24, 2024
13920ab
Replace assert which doesn't take in account presence of invalid down…
knizhnik Nov 25, 2024
69eb618
Add extra checks for IOS prefetch
knizhnik Nov 26, 2024
f27dbe1
Do not use stack->bts_offset because content of parent page can be ch…
knizhnik Nov 26, 2024
c2f65b3
Remove unintended changes
knizhnik Dec 12, 2024
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40 changes: 27 additions & 13 deletions src/backend/access/nbtree/nbtsearch.c
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Original file line number Diff line number Diff line change
Expand Up @@ -848,9 +848,8 @@ _bt_compare(Relation rel,

/*
* _bt_read_parent_for_prefetch - read parent page and extract references to children for prefetch.
* This functions returns offset of first item.
*/
static int
static void
_bt_read_parent_for_prefetch(IndexScanDesc scan, BlockNumber parent, ScanDirection dir)
{
Relation rel = scan->indexRelation;
Expand Down Expand Up @@ -880,6 +879,7 @@ _bt_read_parent_for_prefetch(IndexScanDesc scan, BlockNumber parent, ScanDirecti
so->next_parent = opaque->btpo_next;
if (so->next_parent == P_NONE)
next_parent_prefetch_index = -1;

for (i = 0, j = 0; i < n_child; i++)
{
ItemId itemid = PageGetItemId(page, offnum + i);
Expand All @@ -894,6 +894,7 @@ _bt_read_parent_for_prefetch(IndexScanDesc scan, BlockNumber parent, ScanDirecti
so->next_parent = opaque->btpo_prev;
if (so->next_parent == P_NONE)
next_parent_prefetch_index = -1;

for (i = 0, j = 0; i < n_child; i++)
{
ItemId itemid = PageGetItemId(page, offnum + n_child - i - 1);
Expand All @@ -906,7 +907,6 @@ _bt_read_parent_for_prefetch(IndexScanDesc scan, BlockNumber parent, ScanDirecti
so->n_prefetch_blocks = j;
so->last_prefetch_index = 0;
_bt_relbuf(rel, buf);
return offnum;
}

/*
Expand Down Expand Up @@ -1472,16 +1472,30 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
/* Start prefetching for index only scan */
if (so->prefetch_maximum > 0 && stack != NULL && scan->xs_want_itup) /* index only scan */
{
int first_offset = _bt_read_parent_for_prefetch(scan, stack->bts_blkno, dir);
int skip = ScanDirectionIsForward(dir)
? stack->bts_offset - first_offset
: first_offset + so->n_prefetch_blocks - 1 - stack->bts_offset;
Assert(so->n_prefetch_blocks >= skip);
so->current_prefetch_distance = INCREASE_PREFETCH_DISTANCE_STEP;
so->n_prefetch_requests = Min(so->current_prefetch_distance, so->n_prefetch_blocks - skip);
so->last_prefetch_index = skip + so->n_prefetch_requests;
for (int i = skip; i < so->last_prefetch_index; i++)
PrefetchBuffer(rel, MAIN_FORKNUM, so->prefetch_blocks[i]);
BlockNumber leaf = BufferGetBlockNumber(buf);

_bt_read_parent_for_prefetch(scan, stack->bts_blkno, dir);

/*
* We can not use stack->bts_offset because once parent page is unlocked, it can be updated and state captured by
* by _bt_read_parent_for_prefetch may not match with _bt_search
*/

for (int j = 0; j < so->n_prefetch_blocks; j++)
{
if (so->prefetch_blocks[j] == leaf)
{
so->current_prefetch_distance = INCREASE_PREFETCH_DISTANCE_STEP;
so->n_prefetch_requests = Min(so->current_prefetch_distance, so->n_prefetch_blocks - j);
so->last_prefetch_index = j + so->n_prefetch_requests;

do {
PrefetchBuffer(rel, MAIN_FORKNUM, so->prefetch_blocks[j]);
} while (++j < so->last_prefetch_index);

break;
}
}
}

/* don't need to keep the stack around... */
Expand Down