database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-09-03 15:22:11 +03:00
Code Activity

Improve concurrency of foreign key locking

Browse Source 
This patch introduces two additional lock modes for tuples: "SELECT FOR
KEY SHARE" and "SELECT FOR NO KEY UPDATE".  These don't block each
other, in contrast with already existing "SELECT FOR SHARE" and "SELECT
FOR UPDATE".  UPDATE commands that do not modify the values stored in
the columns that are part of the key of the tuple now grab a SELECT FOR
NO KEY UPDATE lock on the tuple, allowing them to proceed concurrently
with tuple locks of the FOR KEY SHARE variety.

Foreign key triggers now use FOR KEY SHARE instead of FOR SHARE; this
means the concurrency improvement applies to them, which is the whole
point of this patch.

The added tuple lock semantics require some rejiggering of the multixact
module, so that the locking level that each transaction is holding can
be stored alongside its Xid.  Also, multixacts now need to persist
across server restarts and crashes, because they can now represent not
only tuple locks, but also tuple updates.  This means we need more
careful tracking of lifetime of pg_multixact SLRU files; since they now
persist longer, we require more infrastructure to figure out when they
can be removed.  pg_upgrade also needs to be careful to copy
pg_multixact files over from the old server to the new, or at least part
of multixact.c state, depending on the versions of the old and new
servers.

Tuple time qualification rules (HeapTupleSatisfies routines) need to be
careful not to consider tuples with the "is multi" infomask bit set as
being only locked; they might need to look up MultiXact values (i.e.
possibly do pg_multixact I/O) to find out the Xid that updated a tuple,
whereas they previously were assured to only use information readily
available from the tuple header.  This is considered acceptable, because
the extra I/O would involve cases that would previously cause some
commands to block waiting for concurrent transactions to finish.

Another important change is the fact that locking tuples that have
previously been updated causes the future versions to be marked as
locked, too; this is essential for correctness of foreign key checks.
This causes additional WAL-logging, also (there was previously a single
WAL record for a locked tuple; now there are as many as updated copies
of the tuple there exist.)

With all this in place, contention related to tuples being checked by
foreign key rules should be much reduced.

As a bonus, the old behavior that a subtransaction grabbing a stronger
tuple lock than the parent (sub)transaction held on a given tuple and
later aborting caused the weaker lock to be lost, has been fixed.

Many new spec files were added for isolation tester framework, to ensure
overall behavior is sane.  There's probably room for several more tests.

There were several reviewers of this patch; in particular, Noah Misch
and Andres Freund spent considerable time in it.  Original idea for the
patch came from Simon Riggs, after a problem report by Joel Jacobson.
Most code is from me, with contributions from Marti Raudsepp, Alexander
Shulgin, Noah Misch and Andres Freund.

This patch was discussed in several pgsql-hackers threads; the most
important start at the following message-ids:
	AANLkTimo9XVcEzfiBR-ut3KVNDkjm2Vxh+t8kAmWjPuv@mail.gmail.com
	1290721684-sup-3951@alvh.no-ip.org
	1294953201-sup-2099@alvh.no-ip.org
	1320343602-sup-2290@alvh.no-ip.org
	1339690386-sup-8927@alvh.no-ip.org
	4FE5FF020200002500048A3D@gw.wicourts.gov
	4FEAB90A0200002500048B7D@gw.wicourts.gov
This commit is contained in:
Alvaro Herrera
2013-01-23 12:04:59 -03:00
parent f925c79b9f
commit 0ac5ad5134
106 changed files with 6019 additions and 1483 deletions
Download Patch File Download Diff File Expand all files Collapse all files

62
src/include/access/htup_details.h
View File

@@ -162,12 +162,16 @@ struct HeapTupleHeaderData
#define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
#define HEAP_HASOID 0x0008 /* has an object-id field */
/* bit 0x0010 is available */
#define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
#define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */
#define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
#define HEAP_XMAX_SHARED_LOCK 0x0080 /* xmax is shared locker */
/* if either LOCK bit is set, xmax hasn't deleted the tuple, only locked it */
#define HEAP_IS_LOCKED (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_SHARED_LOCK)
#define HEAP_XMAX_LOCK_ONLY 0x0080 /* xmax, if valid, is only a locker */
/* xmax is a shared locker */
#define HEAP_XMAX_SHR_LOCK (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_LOCK_MASK (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
#define HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
#define HEAP_XMAX_COMMITTED 0x0400 /* t_xmax committed */
@@ -182,17 +186,42 @@ struct HeapTupleHeaderData
* upgrade support */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)
#define HEAP_XACT_MASK 0xFFE0 /* visibility-related bits */
#define HEAP_XACT_MASK 0xFFF0 /* visibility-related bits */
/*
* A tuple is only locked (i.e. not updated by its Xmax) if it the
* HEAP_XMAX_LOCK_ONLY bit is set.
*
* See also HeapTupleHeaderIsOnlyLocked, which also checks for a possible
* aborted updater transaction.
*/
#define HEAP_XMAX_IS_LOCKED_ONLY(infomask) \
((infomask) & HEAP_XMAX_LOCK_ONLY)
/*
* Use these to test whether a particular lock is applied to a tuple
*/
#define HEAP_XMAX_IS_SHR_LOCKED(infomask) \
(((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_SHR_LOCK)
#define HEAP_XMAX_IS_EXCL_LOCKED(infomask) \
(((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_EXCL_LOCK)
#define HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) \
(((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_KEYSHR_LOCK)
/* turn these all off when Xmax is to change */
#define HEAP_XMAX_BITS (HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID | \
HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK | HEAP_XMAX_LOCK_ONLY)
/*
* information stored in t_infomask2:
*/
#define HEAP_NATTS_MASK 0x07FF /* 11 bits for number of attributes */
/* bits 0x3800 are available */
/* bits 0x1800 are available */
#define HEAP_KEYS_UPDATED 0x2000 /* tuple was updated and key cols
* modified, or tuple deleted */
#define HEAP_HOT_UPDATED 0x4000 /* tuple was HOT-updated */
#define HEAP_ONLY_TUPLE 0x8000 /* this is heap-only tuple */
#define HEAP2_XACT_MASK 0xC000 /* visibility-related bits */
#define HEAP2_XACT_MASK 0xE000 /* visibility-related bits */
/*
* HEAP_TUPLE_HAS_MATCH is a temporary flag used during hash joins. It is
@@ -219,7 +248,24 @@ struct HeapTupleHeaderData
(tup)->t_choice.t_heap.t_xmin = (xid) \
)
#define HeapTupleHeaderGetXmax(tup) \
/*
* HeapTupleHeaderGetRawXmax gets you the raw Xmax field. To find out the Xid
* that updated a tuple, you might need to resolve the MultiXactId if certain
* bits are set. HeapTupleHeaderGetUpdateXid checks those bits and takes care
* to resolve the MultiXactId if necessary. This might involve multixact I/O,
* so it should only be used if absolutely necessary.
*/
#define HeapTupleHeaderGetUpdateXid(tup) \
( \
(!((tup)->t_infomask & HEAP_XMAX_INVALID) && \
((tup)->t_infomask & HEAP_XMAX_IS_MULTI) && \
!((tup)->t_infomask & HEAP_XMAX_LOCK_ONLY)) ? \
HeapTupleGetUpdateXid(tup) \
: \
HeapTupleHeaderGetRawXmax(tup) \
)
#define HeapTupleHeaderGetRawXmax(tup) \
( \
(tup)->t_choice.t_heap.t_xmax \
)