Implement lazy XID allocation: transactions that do not modify any database

rows will normally never obtain an XID at all.  We already did things this way
for subtransactions, but this patch extends the concept to top-level
transactions.  In applications where there are lots of short read-only
transactions, this should improve performance noticeably; not so much from
removal of the actual XID-assignments, as from reduction of overhead that's
driven by the rate of XID consumption.  We add a concept of a "virtual
transaction ID" so that active transactions can be uniquely identified even
if they don't have a regular XID.  This is a much lighter-weight concept:
uniqueness of VXIDs is only guaranteed over the short term, and no on-disk
record is made about them.

Florian Pflug, with some editorialization by Tom.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 39e27a74924..ebbf8d16262 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1282,7 +1282,7 @@ SELECT viewname, definition FROM pg_views WHERE schemaname <> 'information_schem
  pg_cursors               | SELECT c.name, c.statement, c.is_holdable, c.is_binary, c.is_scrollable, c.creation_time FROM pg_cursor() c(name text, statement text, is_holdable boolean, is_binary boolean, is_scrollable boolean, creation_time timestamp with time zone);
  pg_group                 | SELECT pg_authid.rolname AS groname, pg_authid.oid AS grosysid, ARRAY(SELECT pg_auth_members.member FROM pg_auth_members WHERE (pg_auth_members.roleid = pg_authid.oid)) AS grolist FROM pg_authid WHERE (NOT pg_authid.rolcanlogin);
  pg_indexes               | SELECT n.nspname AS schemaname, c.relname AS tablename, i.relname AS indexname, t.spcname AS tablespace, pg_get_indexdef(i.oid) AS indexdef FROM ((((pg_index x JOIN pg_class c ON ((c.oid = x.indrelid))) JOIN pg_class i ON ((i.oid = x.indexrelid))) LEFT JOIN pg_namespace n ON ((n.oid = c.relnamespace))) LEFT JOIN pg_tablespace t ON ((t.oid = i.reltablespace))) WHERE ((c.relkind = 'r'::"char") AND (i.relkind = 'i'::"char"));
- pg_locks                 | SELECT l.locktype, l.database, l.relation, l.page, l.tuple, l.transactionid, l.classid, l.objid, l.objsubid, l.transaction, l.pid, l.mode, l.granted FROM pg_lock_status() l(locktype text, database oid, relation oid, page integer, tuple smallint, transactionid xid, classid oid, objid oid, objsubid smallint, transaction xid, pid integer, mode text, granted boolean);
+ pg_locks                 | SELECT l.locktype, l.database, l.relation, l.page, l.tuple, l.virtualxid, l.transactionid, l.classid, l.objid, l.objsubid, l.virtualtransaction, l.pid, l.mode, l.granted FROM pg_lock_status() l(locktype text, database oid, relation oid, page integer, tuple smallint, virtualxid text, transactionid xid, classid oid, objid oid, objsubid smallint, virtualtransaction text, pid integer, mode text, granted boolean);
  pg_prepared_statements   | SELECT p.name, p.statement, p.prepare_time, p.parameter_types, p.from_sql FROM pg_prepared_statement() p(name text, statement text, prepare_time timestamp with time zone, parameter_types regtype[], from_sql boolean);
  pg_prepared_xacts        | SELECT p.transaction, p.gid, p.prepared, u.rolname AS owner, d.datname AS database FROM ((pg_prepared_xact() p(transaction xid, gid text, prepared timestamp with time zone, ownerid oid, dbid oid) LEFT JOIN pg_authid u ON ((p.ownerid = u.oid))) LEFT JOIN pg_database d ON ((p.dbid = d.oid)));
  pg_roles                 | SELECT pg_authid.rolname, pg_authid.rolsuper, pg_authid.rolinherit, pg_authid.rolcreaterole, pg_authid.rolcreatedb, pg_authid.rolcatupdate, pg_authid.rolcanlogin, pg_authid.rolconnlimit, '********'::text AS rolpassword, pg_authid.rolvaliduntil, pg_authid.rolconfig, pg_authid.oid FROM pg_authid;