[PATCH] bigint txids vs 'xid' type, new txid_recent(bigint) => xid

From: Craig Ringer <craig(at)2ndquadrant(dot)com>
To: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: [PATCH] bigint txids vs 'xid' type, new txid_recent(bigint) => xid
Date: 2016-08-16 09:15:54
Message-ID: CAMsr+YFDZMN_iZ7KrRoe+j0KVLQvFVgvZxbcVxR-MLjgtoZugA@mail.gmail.com
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Hi all

While implementing support for traceable transactions (finding out after
the fact whether an xact committed or aborted), I've found that Pg is very
inconsistent with what it considers a transaction ID from a user facing
point of view, to the point where I think it's hard for users to write
correct queries.

txid_current() returns a 64-bit xid in which the higher 32 bits are the xid
epoch. This providers users with wraparound protection and means they don't
have to deal with the moving xid threshold.

Many other functions accept and return 'xid', the 32-bit type that isn't
wraparound protected. Presumably they assume you'll only use them with
recent transaction IDs, but there are a couple of problems with this:

* We can't ensure they're only used with recent XIDs and can't detect if
they're passed a wrapped around xid

* There's no good way to _get_ a 32-bit xid for the current xact since
txid_current() returns a 64-bit bigint xid.

(I have to admit that in the past I always blindly assumed that
txid_current() returned bigint for historical reasons, because we don't
have a uint32 type and the xid type didn't exist yet. So I'd do things like
get the value of txid_current() and pass it to pg_xact_commit_timestamp()
later on. This turns out to be wrong, it just happens to work until the
epoch counter increments for the first time. Similarly, working around the
seeming oversight of a missing bigint to xid cast with ::text::xid is wrong
but will seem fine at first.)

I'm surprised the 32-bit xid was ever exposed to the user, rather than a
64-bit epoch-extended xid.

It's not clear to me how a user is supposed to correctly pass the result of
txid_current() to anything like pg_xact_commit_timestamp(xid). They'd have
to get the epoch from a new txid_current() call, split both into two 32-bit
values, and do wraparound checking. Exceedingly unwieldy and hard to get
right.

Since I don't think we can get rid of the 32-bit xid, I think we need a
function to get the 32-bit xid from a 64-bit epoch-and-xid with wraparound
protection.

Here's a patch for that, adding a function txid_recent(bigint) => xid that
returns the low 32 bits of a 64-bit xid like that returned from
txid_current if the xid isn't wrapped around. If it's past the wraparound
threshold the function returns null, since most functions that take xid are
strict and will in turn return null. The alternative, an ERROR, seems
harder for users to handle without resorting to plpgsql. It does ERROR on
XIDs in the future though, since there's no good reason to see those. The
epoch is ignored for permanent XIDs.

I don't like the name much, but haven't come up with a better one yet.

Thoughts?

IMO some functions that take 'xid' should be considered for a bigint
variant:

age (as txid_age(bigint))
pg_xact_commit_timestamp

[ select proname from pg_proc where 'xid'::regtype = ANY
(proargtypes::regtype[]) ; ]

and most or all the system views that expose xid should switch to bigint
for 10.0:

pg_class.relfrozenxid
pg_class.relminmxid
pg_database.datfrozenxid
pg_database.datminmxid
pg_locks.transactionid
pg_prepared_xacts.transaction
pg_stat_activity.backend_xid
pg_stat_activity.backend_xmin
pg_stat_replication.backend_xmin
pg_replication_slots.xmin
pg_replication_slots.catalog_xmin

[ select attrelid::regclass || '.' || attname from pg_attribute where
atttypid = 'xid'::regtype AND attnum >= 0; ]

... or if folks find using bigint too ugly, a new xid64 type. "bigxid"?

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
0001-Add-txid_recent-bigint-xid.patch text/x-patch 8.4 KB

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