我正在广泛使用 Rails 4 的模板缓存功能。许多嵌套模板和touch: true许多模型。总的来说，它已被证明是一个易于推理的综合解决方案。
我最近实现了一个功能，其中创建了多个后台作业，这些作业遍历数百个对象并创建与它们相关的其他对象。当创建这些对象中的每一个时，都会触摸同一个用户。
因此，这两个作业并行运行：
- 作业A：连续创建数百个对象，每次触摸用户123
- 作业B：连续创建数百个对象，每次触摸用户123
部署此功能后，我发现有时 postgres 会检测到死锁并取消两个正在等待的查询。错误中显示的查询始终是触摸查询（将用户 123 的 updated_at 更新为当前时间）。
pid: 33541 tid: 0 vtid: LOG: database system was shut down at 2014-03-27 19:57:11 EDT
pid: 33545 tid: 0 vtid: LOG: autovacuum launcher started
pid: 33539 tid: 0 vtid: LOG: database system is ready to accept connections
pid: 33626 tid: 0 vtid: 2/5 LOG: statement: set client_encoding to 'UTF8'
pid: 33626 tid: 0 vtid: 2/6 LOG: statement: set client_encoding to 'unicode'
pid: 33626 tid: 0 vtid: 2/7 LOG: statement: SET client_min_messages TO 'warning'
pid: 33626 tid: 0 vtid: 2/8 LOG: statement: SHOW client_min_messages
pid: 33626 tid: 0 vtid: 2/9 LOG: statement: SET client_min_messages TO 'panic'
pid: 33626 tid: 0 vtid: 2/10 LOG: statement: SET standard_conforming_strings = on
pid: 33626 tid: 0 vtid: 2/11 LOG: statement: SET client_min_messages TO 'warning'
pid: 33626 tid: 0 vtid: 2/12 LOG: statement: SET time zone 'UTC'
pid: 33626 tid: 0 vtid: 2/13 LOG: statement: SELECT oid, typname, typelem, typdelim, typinput FROM pg_type
pid: 33626 tid: 0 vtid: 2/14 LOG: statement: SHOW TIME ZONE
pid: 33626 tid: 0 vtid: 2/15 LOG: statement: SELECT 1
pid: 33626 tid: 0 vtid: 2/16 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'users'
AND n.nspname = ANY (current_schemas(false))
pid: 33626 tid: 0 vtid: 2/17 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"users"'::regclass
pid: 33626 tid: 0 vtid: 2/18 LOG: statement: SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1
pid: 33626 tid: 0 vtid: 2/19 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"users"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/2 LOG: statement: set client_encoding to 'UTF8'
pid: 33627 tid: 0 vtid: 3/3 LOG: statement: set client_encoding to 'unicode'
pid: 33627 tid: 0 vtid: 3/4 LOG: statement: SET client_min_messages TO 'warning'
pid: 33627 tid: 0 vtid: 3/5 LOG: statement: SHOW client_min_messages
pid: 33627 tid: 0 vtid: 3/6 LOG: statement: SET client_min_messages TO 'panic'
pid: 33627 tid: 0 vtid: 3/7 LOG: statement: SET standard_conforming_strings = on
pid: 33627 tid: 0 vtid: 3/8 LOG: statement: SET client_min_messages TO 'warning'
pid: 33627 tid: 0 vtid: 3/9 LOG: statement: SET time zone 'UTC'
pid: 33627 tid: 0 vtid: 3/10 LOG: statement: SELECT oid, typname, typelem, typdelim, typinput FROM pg_type
pid: 33627 tid: 0 vtid: 3/11 LOG: statement: SHOW TIME ZONE
pid: 33627 tid: 0 vtid: 3/12 LOG: statement: SELECT 1
pid: 33627 tid: 0 vtid: 3/13 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"users"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/14 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'users'
AND n.nspname = ANY (current_schemas(false))
pid: 33627 tid: 0 vtid: 3/15 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"users"'::regclass
pid: 33627 tid: 0 vtid: 3/16 LOG: statement: SELECT id FROM "users" WHERE (bared_at IS NOT NULL) ORDER BY "users"."id" ASC LIMIT 1000
pid: 33628 tid: 0 vtid: 4/2 LOG: statement: set client_encoding to 'UTF8'
pid: 33628 tid: 0 vtid: 4/3 LOG: statement: set client_encoding to 'unicode'
pid: 33628 tid: 0 vtid: 4/4 LOG: statement: SET client_min_messages TO 'warning'
pid: 33628 tid: 0 vtid: 4/5 LOG: statement: SHOW client_min_messages
pid: 33628 tid: 0 vtid: 4/6 LOG: statement: SET client_min_messages TO 'panic'
pid: 33628 tid: 0 vtid: 4/7 LOG: statement: SET standard_conforming_strings = on
pid: 33628 tid: 0 vtid: 4/8 LOG: statement: SET client_min_messages TO 'warning'
pid: 33628 tid: 0 vtid: 4/9 LOG: statement: SET time zone 'UTC'
pid: 33628 tid: 0 vtid: 4/10 LOG: statement: SELECT oid, typname, typelem, typdelim, typinput FROM pg_type
pid: 33628 tid: 0 vtid: 4/11 LOG: statement: SHOW TIME ZONE
pid: 33628 tid: 0 vtid: 4/12 LOG: statement: SELECT 1
pid: 33628 tid: 0 vtid: 4/13 LOG: statement: SELECT id FROM "users" WHERE (bared_at IS NOT NULL) ORDER BY "users"."id" ASC LIMIT 1000
pid: 33627 tid: 0 vtid: 3/17 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'monkeys'
AND n.nspname = ANY (current_schemas(false))
pid: 33628 tid: 0 vtid: 4/14 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'monkeys'
AND n.nspname = ANY (current_schemas(false))
pid: 33627 tid: 0 vtid: 3/18 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"monkeys"'::regclass
pid: 33628 tid: 0 vtid: 4/15 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"monkeys"'::regclass
pid: 33627 tid: 0 vtid: 3/19 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"users_monkeys"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33628 tid: 0 vtid: 4/16 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"users_monkeys"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/20 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"monkeys"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33628 tid: 0 vtid: 4/17 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"monkeys"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/21 LOG: statement: SHOW search_path
pid: 33628 tid: 0 vtid: 4/18 LOG: statement: SHOW search_path
pid: 33627 tid: 0 vtid: 3/23 LOG: execute a1: SELECT COUNT(*) FROM "monkeys" INNER JOIN "users_monkeys" ON "monkeys"."id" = "users_monkeys"."monkey_id" WHERE "users_monkeys"."user_id" = $1 AND (((monkeys.name = 'recruiter') AND (monkeys.resource_type IS NULL) AND (monkeys.resource_id IS NULL)))
pid: 33627 tid: 0 vtid: 3/23 DETAIL: parameters: $1 = '2'
pid: 33628 tid: 0 vtid: 4/20 LOG: execute a1: SELECT COUNT(*) FROM "monkeys" INNER JOIN "users_monkeys" ON "monkeys"."id" = "users_monkeys"."monkey_id" WHERE "users_monkeys"."user_id" = $1 AND (((monkeys.name = 'recruiter') AND (monkeys.resource_type IS NULL) AND (monkeys.resource_id IS NULL)))
pid: 33628 tid: 0 vtid: 4/20 DETAIL: parameters: $1 = '2'
pid: 33627 tid: 0 vtid: 3/24 LOG: statement: SELECT "foos".* FROM "foos" WHERE "foos"."user_id" = 2 AND "foos"."fooer_id" = 1 LIMIT 1
pid: 33628 tid: 0 vtid: 4/21 LOG: statement: SELECT "foos".* FROM "foos" WHERE "foos"."user_id" = 1 AND "foos"."fooer_id" = 2 LIMIT 1
pid: 33628 tid: 0 vtid: 4/22 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"foos"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/25 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"foos"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33628 tid: 0 vtid: 4/23 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'foos'
AND n.nspname = ANY (current_schemas(false))
pid: 33627 tid: 0 vtid: 3/26 LOG: statement: SELECT COUNT(*)
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind in ('v','r')
AND c.relname = 'foos'
AND n.nspname = ANY (current_schemas(false))
pid: 33628 tid: 0 vtid: 4/24 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"foos"'::regclass
pid: 33627 tid: 0 vtid: 3/27 LOG: statement: SELECT attr.attname
FROM pg_attribute attr
INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
WHERE cons.contype = 'p'
AND cons.conrelid = '"foos"'::regclass
pid: 33628 tid: 0 vtid: 4/25 LOG: statement: BEGIN
pid: 33627 tid: 0 vtid: 3/28 LOG: statement: BEGIN
pid: 33628 tid: 0 vtid: 4/25 LOG: statement: SELECT 1 AS one FROM "foos" WHERE ("foos"."user_id" = 1 AND "foos"."id" != 4 AND "foos"."fooer_id" = 2) LIMIT 1
pid: 33627 tid: 0 vtid: 3/28 LOG: statement: SELECT 1 AS one FROM "foos" WHERE ("foos"."user_id" = 2 AND "foos"."id" != 3 AND "foos"."fooer_id" = 1) LIMIT 1
pid: 33628 tid: 0 vtid: 4/25 LOG: statement: SELECT a.attname, format_type(a.atttypid, a.atttypmod),
pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
FROM pg_attribute a LEFT JOIN pg_attrdef d
ON a.attrelid = d.adrelid AND a.attnum = d.adnum
WHERE a.attrelid = '"users"'::regclass
AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum
pid: 33627 tid: 0 vtid: 3/28 LOG: execute a2: SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1
pid: 33627 tid: 0 vtid: 3/28 DETAIL: parameters: $1 = '2'
pid: 33627 tid: 0 vtid: 3/28 LOG: statement: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.619282' WHERE "users"."id" = 2
pid: 33628 tid: 0 vtid: 4/25 LOG: execute a2: SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1
pid: 33628 tid: 0 vtid: 4/25 DETAIL: parameters: $1 = '1'
pid: 33627 tid: 6723 vtid: 3/28 LOG: execute a2: SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1
pid: 33627 tid: 6723 vtid: 3/28 DETAIL: parameters: $1 = '1'
pid: 33628 tid: 0 vtid: 4/25 LOG: statement: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.627175' WHERE "users"."id" = 1
pid: 33627 tid: 6723 vtid: 3/28 LOG: statement: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.628983' WHERE "users"."id" = 1
pid: 33628 tid: 6724 vtid: 4/25 LOG: execute a2: SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1
pid: 33628 tid: 6724 vtid: 4/25 DETAIL: parameters: $1 = '2'
pid: 33628 tid: 6724 vtid: 4/25 LOG: statement: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.632111' WHERE "users"."id" = 2
pid: 33627 tid: 6723 vtid: 3/28 ERROR: deadlock detected
pid: 33627 tid: 6723 vtid: 3/28 DETAIL: Process 33627 waits for ShareLock on transaction 6724; blocked by process 33628.
Process 33628 waits for ShareLock on transaction 6723; blocked by process 33627.
Process 33627: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.628983' WHERE "users"."id" = 1
Process 33628: UPDATE "users" SET "updated_at" = '2014-03-27 23:58:02.632111' WHERE "users"."id" = 2
我很惊讶发生这种死锁，因为行和表都没有被锁定。第二个事务不应该只是等待第一个事务完成吗？
更新：此处只复制最小的 sql 错误：https ://gist.github.com/jjb/9823023