Postgresql 同时刷新物化视图会导致表膨胀_Postgresql_Postgresql 9.5_Materialized Views_Vacuum_Autovacuum

Postgresql 同时刷新物化视图会导致表膨胀

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Postgresql 同时刷新物化视图会导致表膨胀,postgresql,postgresql-9.5,materialized-views,vacuum,autovacuum,Postgresql,Postgresql 9.5,Materialized Views,Vacuum,Autovacuum,在PostgreSQL 9.5中，我决定创建一个物化视图“效果”，并计划每小时进行一次并发刷新，因为我希望它始终可用： REFRESH MATERIALIZED VIEW CONCURRENTLY effects; 一开始一切都很好，我的物化视图正在刷新，磁盘空间使用量或多或少保持不变问题但一段时间后，磁盘使用量开始线性增长我得出的结论是，这种增长的原因是物化视图，并从answer运行查询以获得以下结果： what | by

在PostgreSQL 9.5中，我决定创建一个物化视图“效果”，并计划每小时进行一次并发刷新，因为我希望它始终可用：

REFRESH MATERIALIZED VIEW CONCURRENTLY effects;

一开始一切都很好，我的物化视图正在刷新，磁盘空间使用量或多或少保持不变

问题但一段时间后，磁盘使用量开始线性增长

我得出的结论是，这种增长的原因是物化视图，并从answer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，通过运行以下命令，我发现上一次自动清空此表是在两天前：

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

vacuum（VERBOSE）effects

。它运行了大约半个小时，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

在这一点上，我认为问题已经解决，并开始思考什么会干扰自动真空。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，但令我惊讶的是，它没有改变

只有在我调用

刷新物化视图效果之后不同时执行。直到现在，用于检查表大小的查询的输出才是：
               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)

一切都恢复正常了

问题
问题解决了，但仍有相当多的混乱
谁能解释一下我遇到了什么问题
我今后如何避免这种情况
首先，让我们解释一下膨胀的原因
REFRESH-MATERIALIZED concurrence
在中实现，注释很有启发性：
/*
 * 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，通过从临时表中删除行并插入新行来刷新物化视图。这当然会导致死元组和表膨胀，您的真空（VERBOSE）
输出证实了这一点
在某种程度上，这是您同时为支付的价格
其次，让我们揭穿VACUUM无法移除死元组的神话
VACUUM
将移除死行，但它无法减少膨胀（这可以通过VACUUM（FULL）
实现，但这将锁定视图，就像刷新物化视图
而不同时一样
我怀疑您用来确定死元组数量的查询只是一个估计，它错误地估计了死元组的数量
一个例子说明了这一切
使用pgstattuple
扩展名精确测量表膨胀：
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，刷新并再次测量：
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<代码>真空
去除这些：
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

死元组消失了，但现在有很多空白。
我将添加到上面提供的@Laurenz-Albe完整答案中。还有另一种可能导致腹胀。考虑下面的场景：
您的视图在大多数情况下很少更改（1000000条记录，每个请求更改100条记录），但仍然会得到500000个死元组。其原因在索引列中可以为null
如上所述，当视图同时物化时，将重新创建并比较副本。比较使用强制的唯一索引，但是空值呢？在sql中，空值永远不会彼此相等。因此，如果主键允许空值，则即使未更改也会始终重新创建包含空值的记录并将其添加到表中
为了解决这个问题，您可以添加额外的列，将空列合并到一些从未使用过的值（-1，to_timestamp（0），…），并仅将此列用于主索引
，谢谢您的详细回答！“我怀疑你用来确定死元组数量的查询只是一个估计，它错误地估计了死元组的数量。”-那么你是说我的真空冗余效应
实际上删除了死元组，但没有减少表消耗的空间吗？“真空可以去除死排，但不能减少膨胀”-有没有办法在不锁定的情况下减少膨胀？在膨胀和锁定之间进行选择使得mat_视图在我的例子中毫无用处。此外，膨胀开始“突然”发生的原因还不太清楚。在某种程度上，视图同时刷新，删除的行被释放，只是在某个时候它开始不断增长。你明白我的意思。有像pg_squence
和pgrepack
这样的工具可以在没有VACUUM（FULL）
的情况下重新组织表格。即使有一些膨胀，物化视图也不应该无限增长，因为（正常）真空后的空白空间将被重用。从我的案例中看到，它确实线性增长，无限增长。超过100+GB，而不是1 GB。也许这是一种特殊情况，意识到这种可能性是件好事。也许是其他条件触发了它。我决定放弃物化视图方法，因为我每次扫描的表只插入到其中-因此我可以通过增量添加来进行刷新（节省大量工作负载并避免上述问题）。也许您的数据库中有某种东西会阻止自动真空长事务，准备好的事务或复制插槽。
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