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Enhance nbtree index tuple deletion.

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Teach nbtree and heapam to cooperate in order to eagerly remove
duplicate tuples representing dead MVCC versions.  This is "bottom-up
deletion".  Each bottom-up deletion pass is triggered lazily in response
to a flood of versions on an nbtree leaf page.  This usually involves a
"logically unchanged index" hint (these are produced by the executor
mechanism added by commit 9dc718bd).

The immediate goal of bottom-up index deletion is to avoid "unnecessary"
page splits caused entirely by version duplicates.  It naturally has an
even more useful effect, though: it acts as a backstop against
accumulating an excessive number of index tuple versions for any given
_logical row_.  Bottom-up index deletion complements what we might now
call "top-down index deletion": index vacuuming performed by VACUUM.
Bottom-up index deletion responds to the immediate local needs of
queries, while leaving it up to autovacuum to perform infrequent clean
sweeps of the index.  The overall effect is to avoid certain
pathological performance issues related to "version churn" from UPDATEs.

The previous tableam interface used by index AMs to perform tuple
deletion (the table_compute_xid_horizon_for_tuples() function) has been
replaced with a new interface that supports certain new requirements.
Many (perhaps all) of the capabilities added to nbtree by this commit
could also be extended to other index AMs.  That is left as work for a
later commit.

Extend deletion of LP_DEAD-marked index tuples in nbtree by adding logic
to consider extra index tuples (that are not LP_DEAD-marked) for
deletion in passing.  This increases the number of index tuples deleted
significantly in many cases.  The LP_DEAD deletion process (which is now
called "simple deletion" to clearly distinguish it from bottom-up
deletion) won't usually need to visit any extra table blocks to check
these extra tuples.  We have to visit the same table blocks anyway to
generate a latestRemovedXid value (at least in the common case where the
index deletion operation's WAL record needs such a value).

Testing has shown that the "extra tuples" simple deletion enhancement
increases the number of index tuples deleted with almost any workload
that has LP_DEAD bits set in leaf pages.  That is, it almost never fails
to delete at least a few extra index tuples.  It helps most of all in
cases that happen to naturally have a lot of delete-safe tuples.  It's
not uncommon for an individual deletion operation to end up deleting an
order of magnitude more index tuples compared to the old naive approach
(e.g., custom instrumentation of the patch shows that this happens
fairly often when the regression tests are run).

Add a further enhancement that augments simple deletion and bottom-up
deletion in indexes that make use of deduplication: Teach nbtree's
_bt_delitems_delete() function to support granular TID deletion in
posting list tuples.  It is now possible to delete individual TIDs from
posting list tuples provided the TIDs have a tableam block number of a
table block that gets visited as part of the deletion process (visiting
the table block can be triggered directly or indirectly).  Setting the
LP_DEAD bit of a posting list tuple is still an all-or-nothing thing,
but that matters much less now that deletion only needs to start out
with the right _general_ idea about which index tuples are deletable.

Bump XLOG_PAGE_MAGIC because xl_btree_delete changed.

No bump in BTREE_VERSION, since there are no changes to the on-disk
representation of nbtree indexes.  Indexes built on PostgreSQL 12 or
PostgreSQL 13 will automatically benefit from bottom-up index deletion
(i.e. no reindexing required) following a pg_upgrade.  The enhancement
to simple deletion is available with all B-Tree indexes following a
pg_upgrade, no matter what PostgreSQL version the user upgrades from.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wzm+maE3apHB8NOtmM=p-DO65j2V5GzAWCOEEuy3JZgb2g@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2021-01-13 09:21:32 -08:00
parent 9dc718bdf2
commit d168b66682
19 changed files with 2120 additions and 450 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
src/include/access/heapam.h
View File

@@ -166,9 +166,8 @@ extern void simple_heap_delete(Relation relation, ItemPointer tid);
extern void simple_heap_update(Relation relation, ItemPointer otid,
HeapTuple tup);
extern TransactionId heap_compute_xid_horizon_for_tuples(Relation rel,
ItemPointerData *items,
int nitems);
extern TransactionId heap_index_delete_tuples(Relation rel,
TM_IndexDeleteOp *delstate);
/* in heap/pruneheap.c */
struct GlobalVisState;

19
src/include/access/nbtree.h
View File

@@ -17,6 +17,7 @@
#include "access/amapi.h"
#include "access/itup.h"
#include "access/sdir.h"
#include "access/tableam.h"
#include "access/xlogreader.h"
#include "catalog/pg_am_d.h"
#include "catalog/pg_index.h"
@@ -168,7 +169,7 @@ typedef struct BTMetaPageData
/*
* MaxTIDsPerBTreePage is an upper bound on the number of heap TIDs tuples
* that may be stored on a btree leaf page. It is used to size the
* per-page temporary buffers used by index scans.
* per-page temporary buffers.
*
* Note: we don't bother considering per-tuple overheads here to keep
* things simple (value is based on how many elements a single array of
@@ -766,8 +767,9 @@ typedef struct BTDedupStateData
typedef BTDedupStateData *BTDedupState;
/*
* BTVacuumPostingData is state that represents how to VACUUM a posting list
* tuple when some (though not all) of its TIDs are to be deleted.
* BTVacuumPostingData is state that represents how to VACUUM (or delete) a
* posting list tuple when some (though not all) of its TIDs are to be
* deleted.
*
* Convention is that itup field is the original posting list tuple on input,
* and palloc()'d final tuple used to overwrite existing tuple on output.
@@ -1031,6 +1033,8 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
extern void _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel,
IndexTuple newitem, Size newitemsz,
bool checkingunique);
extern bool _bt_bottomupdel_pass(Relation rel, Buffer buf, Relation heapRel,
Size newitemsz);
extern void _bt_dedup_start_pending(BTDedupState state, IndexTuple base,
OffsetNumber baseoff);
extern bool _bt_dedup_save_htid(BTDedupState state, IndexTuple itup);
@@ -1045,7 +1049,8 @@ extern IndexTuple _bt_swap_posting(IndexTuple newitem, IndexTuple oposting,
* prototypes for functions in nbtinsert.c
*/
extern bool _bt_doinsert(Relation rel, IndexTuple itup,
IndexUniqueCheck checkUnique, Relation heapRel);
IndexUniqueCheck checkUnique, bool indexUnchanged,
Relation heapRel);
extern void _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack);
extern Buffer _bt_getstackbuf(Relation rel, BTStack stack, BlockNumber child);
@@ -1083,9 +1088,9 @@ extern bool _bt_page_recyclable(Page page);
extern void _bt_delitems_vacuum(Relation rel, Buffer buf,
OffsetNumber *deletable, int ndeletable,
BTVacuumPosting *updatable, int nupdatable);
extern void _bt_delitems_delete(Relation rel, Buffer buf,
OffsetNumber *deletable, int ndeletable,
Relation heapRel);
extern void _bt_delitems_delete_check(Relation rel, Buffer buf,
Relation heapRel,
TM_IndexDeleteOp *delstate);
extern uint32 _bt_pagedel(Relation rel, Buffer leafbuf,
TransactionId *oldestBtpoXact);

101
src/include/access/nbtxlog.h
View File

@@ -176,24 +176,6 @@ typedef struct xl_btree_dedup
#define SizeOfBtreeDedup (offsetof(xl_btree_dedup, nintervals) + sizeof(uint16))
/*
* This is what we need to know about delete of individual leaf index tuples.
* The WAL record can represent deletion of any number of index tuples on a
* single index page when *not* executed by VACUUM. Deletion of a subset of
* the TIDs within a posting list tuple is not supported.
*
* Backup Blk 0: index page
*/
typedef struct xl_btree_delete
{
TransactionId latestRemovedXid;
uint32 ndeleted;
/* DELETED TARGET OFFSET NUMBERS FOLLOW */
} xl_btree_delete;
#define SizeOfBtreeDelete (offsetof(xl_btree_delete, ndeleted) + sizeof(uint32))
/*
* This is what we need to know about page reuse within btree. This record
* only exists to generate a conflict point for Hot Standby.
@@ -211,9 +193,61 @@ typedef struct xl_btree_reuse_page
#define SizeOfBtreeReusePage (sizeof(xl_btree_reuse_page))
/*
* This is what we need to know about which TIDs to remove from an individual
* posting list tuple during vacuuming. An array of these may appear at the
* end of xl_btree_vacuum records.
* xl_btree_vacuum and xl_btree_delete records describe deletion of index
* tuples on a leaf page. The former variant is used by VACUUM, while the
* latter variant is used by the ad-hoc deletions that sometimes take place
* when btinsert() is called.
*
* The records are very similar. The only difference is that xl_btree_delete
* has to include a latestRemovedXid field to generate recovery conflicts.
* (VACUUM operations can just rely on earlier conflicts generated during
* pruning of the table whose TIDs the to-be-deleted index tuples point to.
* There are also small differences between each REDO routine that we don't go
* into here.)
*
* xl_btree_vacuum and xl_btree_delete both represent deletion of any number
* of index tuples on a single leaf page using page offset numbers. Both also
* support "updates" of index tuples, which is how deletes of a subset of TIDs
* contained in an existing posting list tuple are implemented.
*
* Updated posting list tuples are represented using xl_btree_update metadata.
* The REDO routines each use the xl_btree_update entries (plus each
* corresponding original index tuple from the target leaf page) to generate
* the final updated tuple.
*
* Updates are only used when there will be some remaining TIDs left by the
* REDO routine. Otherwise the posting list tuple just gets deleted outright.
*/
typedef struct xl_btree_vacuum
{
uint16 ndeleted;
uint16 nupdated;
/* DELETED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TUPLES METADATA (xl_btree_update) ARRAY FOLLOWS */
} xl_btree_vacuum;
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, nupdated) + sizeof(uint16))
typedef struct xl_btree_delete
{
TransactionId latestRemovedXid;
uint16 ndeleted;
uint16 nupdated;
/* DELETED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TUPLES METADATA (xl_btree_update) ARRAY FOLLOWS */
} xl_btree_delete;
#define SizeOfBtreeDelete (offsetof(xl_btree_delete, nupdated) + sizeof(uint16))
/*
* The offsets that appear in xl_btree_update metadata are offsets into the
* original posting list from tuple, not page offset numbers. These are
* 0-based. The page offset number for the original posting list tuple comes
* from the main xl_btree_vacuum/xl_btree_delete record.
*/
typedef struct xl_btree_update
{
@@ -224,31 +258,6 @@ typedef struct xl_btree_update
#define SizeOfBtreeUpdate (offsetof(xl_btree_update, ndeletedtids) + sizeof(uint16))
/*
* This is what we need to know about a VACUUM of a leaf page. The WAL record
* can represent deletion of any number of index tuples on a single index page
* when executed by VACUUM. It can also support "updates" of index tuples,
* which is how deletes of a subset of TIDs contained in an existing posting
* list tuple are implemented. (Updates are only used when there will be some
* remaining TIDs once VACUUM finishes; otherwise the posting list tuple can
* just be deleted).
*
* Updated posting list tuples are represented using xl_btree_update metadata.
* The REDO routine uses each xl_btree_update (plus its corresponding original
* index tuple from the target leaf page) to generate the final updated tuple.
*/
typedef struct xl_btree_vacuum
{
uint16 ndeleted;
uint16 nupdated;
/* DELETED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TARGET OFFSET NUMBERS FOLLOW */
/* UPDATED TUPLES METADATA ARRAY FOLLOWS */
} xl_btree_vacuum;
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, nupdated) + sizeof(uint16))
/*
* This is what we need to know about marking an empty subtree for deletion.
* The target identifies the tuple removed from the parent page (note that we

128
src/include/access/tableam.h
View File

@@ -128,6 +128,106 @@ typedef struct TM_FailureData
bool traversed;
} TM_FailureData;
/*
* State used when calling table_index_delete_tuples().
*
* Represents the status of table tuples, referenced by table TID and taken by
* index AM from index tuples. State consists of high level parameters of the
* deletion operation, plus two mutable palloc()'d arrays for information
* about the status of individual table tuples. These are conceptually one
* single array. Using two arrays keeps the TM_IndexDelete struct small,
* which makes sorting the first array (the deltids array) fast.
*
* Some index AM callers perform simple index tuple deletion (by specifying
* bottomup = false), and include only known-dead deltids. These known-dead
* entries are all marked knowndeletable = true directly (typically these are
* TIDs from LP_DEAD-marked index tuples), but that isn't strictly required.
*
* Callers that specify bottomup = true are "bottom-up index deletion"
* callers. The considerations for the tableam are more subtle with these
* callers because they ask the tableam to perform highly speculative work,
* and might only expect the tableam to check a small fraction of all entries.
* Caller is not allowed to specify knowndeletable = true for any entry
* because everything is highly speculative. Bottom-up caller provides
* context and hints to tableam -- see comments below for details on how index
* AMs and tableams should coordinate during bottom-up index deletion.
*
* Simple index deletion callers may ask the tableam to perform speculative
* work, too. This is a little like bottom-up deletion, but not too much.
* The tableam will only perform speculative work when it's practically free
* to do so in passing for simple deletion caller (while always performing
* whatever work is is needed to enable knowndeletable/LP_DEAD index tuples to
* be deleted within index AM). This is the real reason why it's possible for
* simple index deletion caller to specify knowndeletable = false up front
* (this means "check if it's possible for me to delete corresponding index
* tuple when it's cheap to do so in passing"). The index AM should only
* include "extra" entries for index tuples whose TIDs point to a table block
* that tableam is expected to have to visit anyway (in the event of a block
* orientated tableam). The tableam isn't strictly obligated to check these
* "extra" TIDs, but a block-based AM should always manage to do so in
* practice.
*
* The final contents of the deltids/status arrays are interesting to callers
* that ask tableam to perform speculative work (i.e. when _any_ items have
* knowndeletable set to false up front). These index AM callers will
* naturally need to consult final state to determine which index tuples are
* in fact deletable.
*
* The index AM can keep track of which index tuple relates to which deltid by
* setting idxoffnum (and/or relying on each entry being uniquely identifiable
* using tid), which is important when the final contents of the array will
* need to be interpreted -- the array can shrink from initial size after
* tableam processing and/or have entries in a new order (tableam may sort
* deltids array for its own reasons). Bottom-up callers may find that final
* ndeltids is 0 on return from call to tableam, in which case no index tuple
* deletions are possible. Simple deletion callers can rely on any entries
* they know to be deletable appearing in the final array as deletable.
*/
typedef struct TM_IndexDelete
{
ItemPointerData tid; /* table TID from index tuple */
int16 id; /* Offset into TM_IndexStatus array */
} TM_IndexDelete;
typedef struct TM_IndexStatus
{
OffsetNumber idxoffnum; /* Index am page offset number */
bool knowndeletable; /* Currently known to be deletable? */
/* Bottom-up index deletion specific fields follow */
bool promising; /* Promising (duplicate) index tuple? */
int16 freespace; /* Space freed in index if deleted */
} TM_IndexStatus;
/*
* Index AM/tableam coordination is central to the design of bottom-up index
* deletion. The index AM provides hints about where to look to the tableam
* by marking some entries as "promising". Index AM does this with duplicate
* index tuples that are strongly suspected to be old versions left behind by
* UPDATEs that did not logically modify indexed values. Index AM may find it
* helpful to only mark entries as promising when they're thought to have been
* affected by such an UPDATE in the recent past.
*
* Bottom-up index deletion casts a wide net at first, usually by including
* all TIDs on a target index page. It is up to the tableam to worry about
* the cost of checking transaction status information. The tableam is in
* control, but needs careful guidance from the index AM. Index AM requests
* that bottomupfreespace target be met, while tableam measures progress
* towards that goal by tallying the per-entry freespace value for known
* deletable entries. (All !bottomup callers can just set these space related
* fields to zero.)
*/
typedef struct TM_IndexDeleteOp
{
bool bottomup; /* Bottom-up (not simple) deletion? */
int bottomupfreespace; /* Bottom-up space target */
/* Mutable per-TID information follows (index AM initializes entries) */
int ndeltids; /* Current # of deltids/status elements */
TM_IndexDelete *deltids;
TM_IndexStatus *status;
} TM_IndexDeleteOp;
/* "options" flag bits for table_tuple_insert */
/* TABLE_INSERT_SKIP_WAL was 0x0001; RelationNeedsWAL() now governs */
#define TABLE_INSERT_SKIP_FSM 0x0002
@@ -342,10 +442,9 @@ typedef struct TableAmRoutine
TupleTableSlot *slot,
Snapshot snapshot);
/* see table_compute_xid_horizon_for_tuples() */
TransactionId (*compute_xid_horizon_for_tuples) (Relation rel,
ItemPointerData *items,
int nitems);
/* see table_index_delete_tuples() */
TransactionId (*index_delete_tuples) (Relation rel,
TM_IndexDeleteOp *delstate);
/* ------------------------------------------------------------------------
@@ -1122,16 +1221,23 @@ table_tuple_satisfies_snapshot(Relation rel, TupleTableSlot *slot,
}
/*
* Compute the newest xid among the tuples pointed to by items. This is used
* to compute what snapshots to conflict with when replaying WAL records for
* page-level index vacuums.
* Determine which index tuples are safe to delete based on their table TID.
*
* Determines which entries from index AM caller's TM_IndexDeleteOp state
* point to vacuumable table tuples. Entries that are found by tableam to be
* vacuumable are naturally safe for index AM to delete, and so get directly
* marked as deletable. See comments above TM_IndexDelete and comments above
* TM_IndexDeleteOp for full details.
*
* Returns a latestRemovedXid transaction ID that caller generally places in
* its index deletion WAL record. This might be used during subsequent REDO
* of the WAL record when in Hot Standby mode -- a recovery conflict for the
* index deletion operation might be required on the standby.
*/
static inline TransactionId
table_compute_xid_horizon_for_tuples(Relation rel,
ItemPointerData *items,
int nitems)
table_index_delete_tuples(Relation rel, TM_IndexDeleteOp *delstate)
{
return rel->rd_tableam->compute_xid_horizon_for_tuples(rel, items, nitems);
return rel->rd_tableam->index_delete_tuples(rel, delstate);
}

2
src/include/access/xlog_internal.h
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@@ -31,7 +31,7 @@
/*
* Each page of XLOG file has a header like this:
*/
#define XLOG_PAGE_MAGIC 0xD108 /* can be used as WAL version indicator */
#define XLOG_PAGE_MAGIC 0xD109 /* can be used as WAL version indicator */
typedef struct XLogPageHeaderData
{