在PostgreSQL 9.5中,我决定创建一个物化视图“效果”并安排每小时并发刷新,因为我希望它始终可用：
REFRESH MATERIALIZED VIEW CONCURRENTLY effects;
一开始一切都运行良好,我的物化视图令人耳目一新,磁盘空间使用率或多或少保持不变.
问题
经过一段时间后,磁盘使用率开始呈线性增长.
我得出结论,这种增长的原因是物化视图并从this回答运行查询以获得以下结果：
what | bytes/ct | bytes_pretty | bytes_per_row
-----------------------------------+-------------+--------------+---------------
core_relation_size | 32224567296 | 30 GB | 21140
visibility_map | 991232 | 968 kB | 0
free_space_map | 7938048 | 7752 kB | 5
table_size_incl_toast | 32233504768 | 30 GB | 21146
indexes_size | 22975922176 | 21 GB | 15073
total_size_incl_toast_and_indexes | 55209426944 | 51 GB | 36220
live_rows_in_text_representation | 316152215 | 302 MB | 207
------------------------------ | | |
row_count | 1524278 | |
live_tuples | 676439 | |
dead_tuples | 1524208 | |
(11 rows)
然后,我发现这个表最后一次自动恢复是两天前,通过运行：
SELECT relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables ORDER BY n_dead_tup desc;
我决定手动调用真空(VERBOSE)效果.它运行了大约半小时,产生了以下输出：
vacuum (VERBOSE) effects;
INFO: vacuuming "public.effects"
INFO: scanned index "effects_idx" to remove 129523454 row versions
DETAIL: CPU 12.16s/55.76u sec elapsed 119.87 sec
INFO: scanned index "effects_campaign_created_idx" to remove 129523454 row versions
DETAIL: CPU 19.11s/154.59u sec elapsed 337.91 sec
INFO: scanned index "effects_campaign_name_idx" to remove 129523454 row versions
DETAIL: CPU 28.51s/151.16u sec elapsed 315.51 sec
INFO: scanned index "effects_campaign_event_type_idx" to remove 129523454 row versions
DETAIL: CPU 38.60s/373.59u sec elapsed 601.73 sec
INFO: "effects": removed 129523454 row versions in 3865537 pages
DETAIL: CPU 59.02s/36.48u sec elapsed 326.43 sec
INFO: index "effects_idx" now contains 1524208 row versions in 472258 pages
DETAIL: 113679000 index row versions were removed.
463896 index pages have been deleted, 60386 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.01 sec.
INFO: index "effects_campaign_created_idx" now contains 1524208 row versions in 664910 pages
DETAIL: 121637488 index row versions were removed.
41014 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: index "effects_campaign_name_idx" now contains 1524208 row versions in 711391 pages
DETAIL: 125650677 index row versions were removed.
696221 index pages have been deleted, 28150 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: index "effects_campaign_event_type_idx" now contains 1524208 row versions in 956018 pages
DETAIL: 127659042 index row versions were removed.
934288 index pages have been deleted, 32105 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "effects": found 0 removable, 493 nonremovable row versions in 3880239 out of 3933663 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 666922 unused item pointers.
Skipped 0 pages due to buffer pins.
0 pages are entirely empty.
CPU 180.49s/788.60u sec elapsed 1799.42 sec.
INFO: vacuuming "pg_toast.pg_toast_1371723"
INFO: index "pg_toast_1371723_index" now contains 0 row versions in 1 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_toast_1371723": found 0 removable, 0 nonremovable row versions in 0 out of 0 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
Skipped 0 pages due to buffer pins.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
VACUUM
此时我认为问题已经解决,并开始考虑可能会干扰autovacuum的问题.可以肯定的是,我再次运行查询以查找该表的空间使用情况,令我惊讶的是它没有改变.
只有在我调用REFRESH MATERIALIZED VIEW效果之后;不是同时发生的.只是现在查询表大小的查询输出是：
what | bytes/ct | bytes_pretty | bytes_per_row
-----------------------------------+-----------+--------------+---------------
core_relation_size | 374005760 | 357 MB | 245
visibility_map | 0 | 0 bytes | 0
free_space_map | 0 | 0 bytes | 0
table_size_incl_toast | 374013952 | 357 MB | 245
indexes_size | 213843968 | 204 MB | 140
total_size_incl_toast_and_indexes | 587857920 | 561 MB | 385
live_rows_in_text_representation | 316175512 | 302 MB | 207
------------------------------ | | |
row_count | 1524385 | |
live_tuples | 676439 | |
dead_tuples | 1524208 | |
(11 rows)
一切都恢复正常……
问题
问题已经解决,但仍然存在相当多的混乱
>有人可以解释一下我遇到的问题吗？
>将来我怎么能避免这种情况？
最佳答案 首先,我们来解释这个膨胀
REFRESH MATERIALIZED CONCURRENTLY在src/backend/commands/matview.c实施,评论很有启发性：
/*
* refresh_by_match_merge
*
* Refresh a materialized view with transactional semantics, while allowing
* concurrent reads.
*
* This is called after a new version of the data has been created in a
* temporary table. It performs a full outer join against the old version of
* the data, producing "diff" results. This join cannot work if there are any
* duplicated rows in either the old or new versions, in the sense that every
* column would compare as equal between the two rows. It does work correctly
* in the face of rows which have at least one NULL value, with all non-NULL
* columns equal. The behavior of NULLs on equality tests and on UNIQUE
* indexes turns out to be quite convenient here; the tests we need to make
* are consistent with default behavior. If there is at least one UNIQUE
* index on the materialized view, we have exactly the guarantee we need.
*
* The temporary table used to hold the diff results contains just the TID of
* the old record (if matched) and the ROW from the new table as a single
* column of complex record type (if matched).
*
* Once we have the diff table, we perform set-based DELETE and INSERT
* operations against the materialized view, and discard both temporary
* tables.
*
* Everything from the generation of the new data to applying the differences
* takes place under cover of an ExclusiveLock, since it seems as though we
* would want to prohibit not only concurrent REFRESH operations, but also
* incremental maintenance. It also doesn't seem reasonable or safe to allow
* SELECT FOR UPDATE or SELECT FOR SHARE on rows being updated or deleted by
* this command.
*/
因此,通过删除行并从临时表中插入新行来刷新物化视图.这当然会导致死元组和表膨胀,这可以通过您的VACUUM(VERBOSE)输出来确认.
在某种程度上,这是你为同时支付的价格.
其次,让我们揭穿VACUUM无法移除死元组的神话
VACUUM将删除死行,但它不能减少膨胀(可以使用VACUUM(FULL)完成,但这会像REFRESH MATERIALIZED VIEW一样锁定视图而不会同时发生).
我怀疑你用来确定死元组数的查询只是一个估计,它会导致死元组的数量错误.
举例说明了这一切
CREATE TABLE tab AS SELECT id, 'row ' || id AS val FROM generate_series(1, 100000) AS id;
-- make sure autovacuum doesn't spoil our demonstration
CREATE MATERIALIZED VIEW tab_v WITH (autovacuum_enabled = off)
AS SELECT * FROM tab;
-- required for CONCURRENTLY
CREATE UNIQUE INDEX ON tab_v (id);
使用pgstattuple扩展来准确测量表膨胀：
CREATE EXTENSION pgstattuple;
SELECT * FROM pgstattuple('tab_v');
-[ RECORD 1 ]------+--------
table_len | 4431872
tuple_count | 100000
tuple_len | 3788895
tuple_percent | 85.49
dead_tuple_count | 0
dead_tuple_len | 0
dead_tuple_percent | 0
free_space | 16724
free_percent | 0.38
现在让我们删除表中的一些行,再次刷新和测量：
DELETE FROM tab WHERE id BETWEEN 40001 AND 80000;
REFRESH MATERIALIZED VIEW CONCURRENTLY tab_v;
SELECT * FROM pgstattuple('tab_v');
-[ RECORD 1 ]------+--------
table_len | 4431872
tuple_count | 60000
tuple_len | 2268895
tuple_percent | 51.19
dead_tuple_count | 40000
dead_tuple_len | 1520000
dead_tuple_percent | 34.3
free_space | 16724
free_percent | 0.38
很多死元组. VACUUM摆脱了这些：
VACUUM tab_v;
SELECT * FROM pgstattuple('tab_v');
-[ RECORD 1 ]------+--------
table_len | 4431872
tuple_count | 60000
tuple_len | 2268895
tuple_percent | 51.19
dead_tuple_count | 0
dead_tuple_len | 0
dead_tuple_percent | 0
free_space | 1616724
free_percent | 36.48
死去的元组消失了,但现在有很多空的空间.
