使用pg_buffercache外掛可檢視shared buffer中的內容.

安裝pg_buffercache
```
[pg12@localhost pg_buffercache]$ make
make -C ../../src/backend generated-headers
make[1]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend'
make -C catalog distprep generated-header-symlinks
make[2]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend/catalog'
make[2]: Nothing to be done for `distprep'.
make[2]: Nothing to be done for `generated-header-symlinks'.
make[2]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend/catalog'
make -C utils distprep generated-header-symlinks
make[2]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend/utils'
make[2]: Nothing to be done for `distprep'.
make[2]: Nothing to be done for `generated-header-symlinks'.
make[2]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend/utils'
make[1]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend'
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -I. -I. -I../../src/include  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o pg_buffercache_pages.o pg_buffercache_pages.c -MMD -MP -MF .deps/pg_buffercache_pages.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -shared -o pg_buffercache.so pg_buffercache_pages.o  -L../../src/port -L../../src/common    -Wl,--as-needed -Wl,-rpath,'/appdb/pg12/pg12beta3/lib',--enable-new-dtags  
[pg12@localhost pg_buffercache]$ sudo make install
[sudo] password for pg12: 
make -C ../../src/backend generated-headers
make[1]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend'
make -C catalog distprep generated-header-symlinks
make[2]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend/catalog'
make[2]: Nothing to be done for `distprep'.
make[2]: Nothing to be done for `generated-header-symlinks'.
make[2]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend/catalog'
make -C utils distprep generated-header-symlinks
make[2]: Entering directory `/home/pg12/source/postgresql-12beta3/src/backend/utils'
make[2]: Nothing to be done for `distprep'.
make[2]: Nothing to be done for `generated-header-symlinks'.
make[2]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend/utils'
make[1]: Leaving directory `/home/pg12/source/postgresql-12beta3/src/backend'
/bin/mkdir -p '/appdb/pg12/pg12beta3/lib/postgresql'
/bin/mkdir -p '/appdb/pg12/pg12beta3/share/postgresql/extension'
/bin/mkdir -p '/appdb/pg12/pg12beta3/share/postgresql/extension'
/bin/install -c -m 755  pg_buffercache.so '/appdb/pg12/pg12beta3/lib/postgresql/pg_buffercache.so'
/bin/install -c -m 644 ./pg_buffercache.control '/appdb/pg12/pg12beta3/share/postgresql/extension/'
/bin/install -c -m 644 ./pg_buffercache--1.2.sql ./pg_buffercache--1.2--1.3.sql ./pg_buffercache--1.1--1.2.sql ./pg_buffercache--1.0--1.1.sql ./pg_buffercache--unpackaged--1.0.sql  '/appdb/pg12/pg12beta3/share/postgresql/extension/'
[pg12@localhost pg_buffercache]$ 
```
簡單使用
```
[local]:5432 pg12@testdb=# create extension pg_buffercache
pg12@testdb-# ;
CREATE EXTENSION
Time: 149.794 ms
[local]:5432 pg12@testdb=# \d pg_buffercache

[local]:5432 pg12@testdb=# 
[local]:5432 pg12@testdb=# select * from pg_buffercache;
-[ RECORD 1 ]----+------
bufferid         | 1
relfilenode      | 33029
reltablespace    | 1664
reldatabase      | 0
relforknumber    | 0
relblocknumber   | 0
isdirty          | f
usagecount       | 5
pinning_backends | 0
-[ RECORD 2 ]----+------
bufferid         | 2
relfilenode      | 32825
reltablespace    | 1664
reldatabase      | 0
relforknumber    | 0
relblocknumber   | 0
isdirty          | f
usagecount       | 4
pinning_backends | 0
...
```
直接查詢pg_buffercache可獲得shared buffer的相關資訊.
建立統計檢視
```
create or replace view vw_buffercache_hogs as
select case
       when pg_buffercache.reldatabase = 0
            then '- global'
       when pg_buffercache.reldatabase <> (select pg_database.oid from pg_database where pg_database.datname = current_database())
            then '- database ' || quote_literal(pg_database.datname)
       when pg_namespace.nspname = 'pg_catalog'
            then '- system catalogues'
       when pg_class.oid is null and pg_buffercache.relfilenode > 0
            then '- unknown file ' || pg_buffercache.relfilenode
       when pg_namespace.nspname = 'pg_toast' and pg_class.relname ~ '^pg_toast_[0-9]+$'
            then (substring(pg_class.relname, 10)::oid)::regclass || ' TOAST'::text
       when pg_namespace.nspname = 'pg_toast' and pg_class.relname ~ '^pg_toast_[0-9]+_index$'
            then ((rtrim(substring(pg_class.relname, 10), '_index'))::oid)::regclass || ' TOAST index'
       else pg_class.oid::regclass::text
       end as key,
       count(*) as buffers, sum(case when pg_buffercache.isdirty then 1 else 0 end) as dirty_buffers,
       round(count(*) / (SELECT pg_settings.setting FROM pg_settings WHERE pg_settings.name = 'shared_buffers')::numeric, 4) as hog_factor
from pg_buffercache
     left join pg_database on pg_database.oid = pg_buffercache.reldatabase
     left join pg_class on pg_class.relfilenode = pg_buffercache.relfilenode
     left join pg_namespace on pg_namespace.oid = pg_class.relnamespace
group by 1
order by 2 desc;

```
查詢該檢視
```
[local]:5432 pg12@testdb=# create or replace view vw_buffercache_hogs as
pg12@testdb-# select case
pg12@testdb-#        when pg_buffercache.reldatabase = 0
pg12@testdb-#             then '- global'
pg12@testdb-#        when pg_buffercache.reldatabase <> (select pg_database.oid from pg_database where pg_database.datname = current_database())
pg12@testdb-#             then '- database ' || quote_literal(pg_database.datname)
pg12@testdb-#        when pg_namespace.nspname = 'pg_catalog'
pg12@testdb-#             then '- system catalogues'
pg12@testdb-#        when pg_class.oid is null and pg_buffercache.relfilenode > 0
pg12@testdb-#             then '- unknown file ' || pg_buffercache.relfilenode
pg12@testdb-#        when pg_namespace.nspname = 'pg_toast' and pg_class.relname ~ '^pg_toast_[0-9]+$'
pg12@testdb-#             then (substring(pg_class.relname, 10)::oid)::regclass || ' TOAST'::text
pg12@testdb-#        when pg_namespace.nspname = 'pg_toast' and pg_class.relname ~ '^pg_toast_[0-9]+_index$'
pg12@testdb-#             then ((rtrim(substring(pg_class.relname, 10), '_index'))::oid)::regclass || ' TOAST index'
pg12@testdb-#        else pg_class.oid::regclass::text
pg12@testdb-#        end as key,
pg12@testdb-#        count(*) as buffers, sum(case when pg_buffercache.isdirty then 1 else 0 end) as dirty_buffers,
pg12@testdb-#        round(count(*) / (SELECT pg_settings.setting FROM pg_settings WHERE pg_settings.name = 'shared_buffers')::numeric, 4) as hog_factor
pg12@testdb-# from pg_buffercache
pg12@testdb-#      left join pg_database on pg_database.oid = pg_buffercache.reldatabase
pg12@testdb-#      left join pg_class on pg_class.relfilenode = pg_buffercache.relfilenode
pg12@testdb-#      left join pg_namespace on pg_namespace.oid = pg_class.relnamespace
pg12@testdb-# group by 1
pg12@testdb-# order by 2 desc;
CREATE VIEW
Time: 69.892 ms
[local]:5432 pg12@testdb=# select * from vw_buffercache_hogs;
           key            | buffers | dirty_buffers | hog_factor 
--------------------------+---------+---------------+------------
                          |   65187 |             0 |     0.9947
 - system catalogues      |     174 |            22 |     0.0027
 - unknown file 32856     |      32 |             1 |     0.0005
 - unknown file 32861     |      28 |             2 |     0.0004
 - global                 |      19 |             0 |     0.0003
 - unknown file 32869     |      15 |             4 |     0.0002
 - unknown file 32868     |      11 |             1 |     0.0002
 t_copy                   |       8 |             0 |     0.0001
 - unknown file 32867     |       8 |             1 |     0.0001
 - unknown file 32860     |       8 |             1 |     0.0001
 - unknown file 32873     |       7 |             2 |     0.0001
 - unknown file 32809     |       7 |             1 |     0.0001
 - unknown file 32816     |       6 |             3 |     0.0001
 - unknown file 32872     |       5 |             1 |     0.0001
 pg_rewrite TOAST         |       4 |             3 |     0.0001
 - unknown file 32815     |       4 |             1 |     0.0001
 - unknown file 32874     |       4 |             1 |     0.0001
 - unknown file 32859     |       3 |             1 |     0.0000
 pg_rewrite TOAST index   |       2 |             1 |     0.0000
 pg_statistic TOAST index |       2 |             0 |     0.0000
 t_import                 |       1 |             0 |     0.0000
 pg_statistic TOAST       |       1 |             0 |     0.0000
(22 rows)

Time: 201.894 ms
```
另外,關於索引建立後,索引資料是否已快取在shared buffer,答案是否定的.
```
[local]:5432 pg12@testdb=# create index idx_t_prewarm_id on t_prewarm(id);
CREATE INDEX
Time: 578.582 ms
[local]:5432 pg12@testdb=# select * from vw_buffercache_hogs;
          key           | buffers | dirty_buffers | hog_factor 
------------------------+---------+---------------+------------
                        |   59920 |             0 |     0.9143
 t_prewarm              |    5406 |             0 |     0.0825
 - system catalogues    |      82 |             6 |     0.0013
 - unknown file 32856   |      32 |             2 |     0.0005
 - unknown file 32861   |      19 |             0 |     0.0003
 - unknown file 32869   |      14 |             1 |     0.0002
 - global               |      14 |             0 |     0.0002
 - unknown file 32867   |       8 |             0 |     0.0001
 - unknown file 32860   |       8 |             1 |     0.0001
 - unknown file 32872   |       5 |             1 |     0.0001
 - unknown file 32873   |       4 |             1 |     0.0001
 - unknown file 32816   |       4 |             0 |     0.0001
 - unknown file 32868   |       4 |             0 |     0.0001
 - unknown file 32859   |       3 |             1 |     0.0000
 - unknown file 32809   |       3 |             0 |     0.0000
 - unknown file 32815   |       3 |             0 |     0.0000
 - unknown file 32874   |       3 |             1 |     0.0000
 pg_rewrite TOAST index |       2 |             0 |     0.0000
 pg_rewrite TOAST       |       2 |             0 |     0.0000
(19 rows)

Time: 221.542 ms

```
在預熱後才會在記憶體中
```
[local]:5432 pg12@testdb=# select pg_prewarm('idx_t_prewarm_id');
 pg_prewarm 
------------
       2745
(1 row)

Time: 51.211 ms
[local]:5432 pg12@testdb=# select * from vw_buffercache_hogs;
          key           | buffers | dirty_buffers | hog_factor 
------------------------+---------+---------------+------------
                        |   62601 |             0 |     0.9552
 idx_t_prewarm_id       |    2745 |             0 |     0.0419
 - system catalogues    |      69 |             0 |     0.0011
 - unknown file 32856   |      31 |             0 |     0.0005
 - unknown file 32861   |      18 |             0 |     0.0003
 - global               |      14 |             0 |     0.0002
 - unknown file 32869   |      11 |             0 |     0.0002
 - unknown file 32860   |       8 |             0 |     0.0001
 - unknown file 32867   |       8 |             0 |     0.0001
 - unknown file 32872   |       5 |             0 |     0.0001
 - unknown file 32816   |       4 |             0 |     0.0001
 t_prewarm              |       4 |             0 |     0.0001
 - unknown file 32873   |       4 |             0 |     0.0001
 - unknown file 32815   |       3 |             0 |     0.0000
 - unknown file 32868   |       3 |             0 |     0.0000
 - unknown file 32809   |       3 |             0 |     0.0000
 pg_rewrite TOAST index |       2 |             0 |     0.0000
 pg_rewrite TOAST       |       2 |             0 |     0.0000
 - unknown file 32874   |       1 |             0 |     0.0000
(19 rows)

Time: 131.575 ms
```

**參考資料**
[Postgresql cache (memory) performance + how to warm up the cache](https://stackoverflow.com/questions/934360/postgresql-cache-memory-performance-how-to-warm-up-the-cache)

PostgreSQL DBA(83) - Extension(pg_buffercache)

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