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，我的实例化视图令人耳目一新，磁盘空间使用或多或少保持不变。
不过，一段时间之后，磁盘使用率开始呈线性增长趋势。
我已经得出结论，这种增长的原因是物化视图，并从this答案中运行查询以得到以下结果：
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，我发现该表的最后一次自动抽真空是在两天前，通过运行：
SELECT relname, n_dead_tup, last_vacuum, last_autovacuum FROM pg_stat_user_tables ORDER BY n_dead_tup desc;
我决定手动致电vacuum (VERBOSE) effects。它运行了大约半个小时，并产生了以下输出：
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，并开始考虑哪些因素会干扰自动真空。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，令我惊讶的是它没有发生变化。
仅在我打电话给REFRESH MATERIALIZED VIEW effects;后不能同时。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)
一切恢复正常...
问题已解决，但仍然存在很多困惑
答案 0 :(得分：2)
REFRESH MATERIALIZED CONCURRENTLY已在src/backend/commands/matview.c中实现，并且注释很有启发性：
/*
* 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，通过删除行并从临时表中插入新行来刷新实例化视图。当然，这可能导致元组死机和表膨胀，这由您的VACUUM (VERBOSE)输出确认。
从某种意义上讲，这就是您为CONCURRENTLY支付的价格。
VACUUM无法删除死元组的神话 VACUUM将删除死行，但不能减少膨胀（可以使用VACUUM (FULL)完成，但这将像REFRESH MATERIALIZED VIEW那样锁定视图而没有{{1} }。
我怀疑您用于确定无效元组数的查询只是一个估计，它使错误的无效元组数成为错误估计。
CONCURRENTLY使用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);
扩展名来准确衡量表格膨胀：
pgstattuple现在让我们删除表中的某些行，刷新并再次测量：
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
很多死元组。 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
摆脱了这些：
VACUUM死掉的元组不见了，但是现在有很多空白。
答案 1 :(得分：2)
我要向@Laurenz Albe添加上面提供的完整答案。腹胀还有另一种可能性。请考虑以下情形：
您认为大多数情况下很少更改（1000000条记录，每个请求更改100条记录），但仍然得到500000个死元组。原因可能是在索引列中为空。
如以上答案中所述，当视图同时实现时，将重新创建并比较副本。比较使用强制性唯一索引，但是null呢？在sql中，null永远不会彼此相等。因此，如果您的主键允许使用空值，那么即使空值不变也包含空值的记录将始终被重新创建并添加到表中
要解决此问题，您可以做的就是消除膨胀，即添加其他列以将null列合并为一些从未使用的值（-1，to_timestamp（0），...），并且仅使用此列主索引