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PostgreSQL 的扩展pg_freespacemap
pg_freespacemap 是 PostgreSQL 提供的一个内置扩展，用于查看表的空闲空间映射（Free Space Map, FSM）信息。这个扩展对于数据库性能调优和空间管理非常有用。
一 扩展概述
功能：提供对表的空闲空间映射的直接访问
用途：
- 分析表的空间利用率
- 识别空间浪费严重的表
- 优化VACUUM和空间回收策略
- 诊断膨胀问题
版本支持：PostgreSQL 8.4+（不同版本功能可能略有差异）
二、安装与启用
-- 创建扩展
CREATE EXTENSION pg_freespacemap;-- 验证是否安装成功
SELECT * FROM pg_available_extensions WHERE name = 'pg_freespacemap';
输出示例：
white=# CREATE EXTENSION pg_freespacemap;
CREATE EXTENSION
white=# SELECT * FROM pg_available_extensions WHERE name = 'pg_freespacemap';name | default_version | installed_version | comment
-----------------+-----------------+-------------------+----------------------------------pg_freespacemap | 1.2 | 1.2 | examine the free space map (FSM)
(1 row)white=#
三 主要功能函数
3.1 pg_freespace(relation regclass, blkno bigint)
返回特定表块的空闲空间字节数
-- 获取表'yewu1.t1'的第0块的空闲空间
SELECT pg_freespace('yewu1.t1'::regclass, 0);
输出示例：
white=# SELECT pg_freespace('yewu1.t1'::regclass, 0);pg_freespace
--------------0
(1 row)white=#
3.2 pg_freespace(relation regclass)
返回表所有块的空闲空间信息
-- 获取表'test'的所有块空闲空间
SELECT * FROM pg_freespace('yewu1.t1'::regclass);
输出示例：
white=# SELECT * FROM pg_freespace('yewu1.t1'::regclass);blkno | avail
-------+-------0 | 0
(1 row)white=#
四 使用示例
示例1：分析表的空间利用率
-- 创建测试表
CREATE TABLE test_table (id serial, data text);
INSERT INTO test_table (data) SELECT generate_series(1,10000)::text;-- 查看空闲空间分布
SELECT blkno, avail
FROM pg_freespace('test_table'::regclass)
ORDER BY avail DESC
LIMIT 10;
输出示例：
white=# CREATE TABLE yewu1.test_table (id serial, data text);
CREATE TABLE
white=# INSERT INTO yewu1.test_table (data) SELECT generate_series(1,10000)::text;
INSERT 0 10000
white=#
white=# -- 查看空闲空间分布
white=# SELECT blkno, avail
white-# FROM pg_freespace('yewu1.test_table'::regclass)
white-# ORDER BY avail DESC
white-# LIMIT 10;blkno | avail
-------+-------4 | 322 | 07 | 09 | 05 | 06 | 03 | 08 | 01 | 00 | 0
(10 rows)white=#
示例2：识别空间浪费严重的表
-- 找出数据库中平均空闲空间最多的表
SELECTn.nspname AS schema,c.relname AS table,pg_size_pretty(pg_relation_size(c.oid)) AS size,(SELECT avg(avail) FROM pg_freespace(c.oid)) AS avg_free_space
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind = 'r'
ORDER BY avg_free_space DESC
LIMIT 10;
输出示例：
white=# SELECT
white-# n.nspname AS schema,
white-# c.relname AS table,
white-# pg_size_pretty(pg_relation_size(c.oid)) AS size,
white-# (SELECT avg(avail) FROM pg_freespace(c.oid)) AS avg_free_space
white-# FROM pg_class c
white-# JOIN pg_namespace n ON n.oid = c.relnamespace
white-# WHERE c.relkind = 'r'
white-# ORDER BY avg_free_space DESC
white-# LIMIT 10;schema | table | size | avg_free_space
------------+-----------------------+---------+----------------pg_catalog | pg_user_mapping | 0 bytes | pg_catalog | pg_subscription | 0 bytes | pg_catalog | pg_statistic_ext_data | 0 bytes | yewu1 | test5 | 0 bytes | pg_catalog | pg_foreign_table | 0 bytes | yewu1 | test6 | 0 bytes | yewu1 | test2 | 0 bytes | yewu1 | test4 | 0 bytes | yewu1 | test3 | 0 bytes | pg_catalog | pg_inherits | 0 bytes |
(10 rows)white=#
五 输出解释
pg_freespace 函数输出
| 列名 | 类型 | 描述 |
|---|---|---|
| blkno | bigint | 块号（从0开始） |
| avail | int | 该块中可用空间字节数 |
六 实际应用场景
场景1：定期空间监控
-- 创建监控视图
CREATE VIEW table_space_monitor AS
SELECTn.nspname AS schema,c.relname AS table,pg_size_pretty(pg_relation_size(c.oid)) AS size,(SELECT avg(avail) FROM pg_freespace(c.oid)) AS avg_free_bytes,round((SELECT sum(avail) FROM pg_freespace(c.oid)) * 100.0 / NULLIF(pg_relation_size(c.oid), 0), 2) AS free_percent
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind = 'r'
AND pg_relation_size(c.oid) > 0;
输出示例：
white=# CREATE VIEW table_space_monitor AS
white-# SELECT
white-# n.nspname AS schema,
white-# c.relname AS table,
white-# pg_size_pretty(pg_relation_size(c.oid)) AS size,
white-# (SELECT avg(avail) FROM pg_freespace(c.oid)) AS avg_free_bytes,
white-# round((SELECT sum(avail) FROM pg_freespace(c.oid)) * 100.0 /
white(# NULLIF(pg_relation_size(c.oid), 0), 2) AS free_percent
white-# FROM pg_class c
white-# JOIN pg_namespace n ON n.oid = c.relnamespace
white-# WHERE c.relkind = 'r'
white-# AND pg_relation_size(c.oid) > 0;
CREATE VIEW
white=#
white=# select * from table_space_monitor;schema | table | size | avg_free_bytes | free_percent
--------------------+-------------------------+------------+------------------------+--------------public | pgbench_accounts | 128 MB | 107.0799220272904483 | 1.31public | pgbench_branches | 8192 bytes | 0.00000000000000000000 | 0.00public | pgbench_history | 13 MB | 4.3969465648854962 | 0.05public | pgbench_tellers | 8192 bytes | 0.00000000000000000000 | 0.00pg_catalog | pg_type | 128 kB | 54.0000000000000000 | 0.66pg_catalog | pg_statistic | 208 kB | 17.2307692307692308 | 0.21pg_catalog | pg_authid | 8192 bytes | 5984.0000000000000000 | 73.05yewu1 | t2 | 440 kB | 140.2181818181818182 | 1.71yewu1 | test10 | 8192 bytes | 0.00000000000000000000 | 0.00pg_catalog | pg_attribute | 568 kB | 1.3521126760563380 | 0.02pg_catalog | pg_proc | 832 kB | 74.1538461538461538 | 0.91pg_catalog | pg_attrdef | 8192 bytes | 0.00000000000000000000 | 0.00yewu1 | t1 | 8192 bytes | 0.00000000000000000000 | 0.00pg_catalog | pg_constraint | 24 kB | 1546.6666666666666667 | 18.88pg_catalog | pg_index | 40 kB | 32.0000000000000000 | 0.39pg_catalog | pg_operator | 112 kB | 162.2857142857142857 | 1.98pg_catalog | pg_opfamily | 16 kB | 272.0000000000000000 | 3.32pg_catalog | pg_opclass | 24 kB | 842.6666666666666667 | 10.29pg_catalog | pg_am | 8192 bytes | 7392.0000000000000000 | 90.23pg_catalog | pg_amop | 56 kB | 59.4285714285714286 | 0.73pg_catalog | pg_amproc | 40 kB | 921.6000000000000000 | 11.25pg_catalog | pg_language | 8192 bytes | 7648.0000000000000000 | 93.36pg_catalog | pg_aggregate | 16 kB | 128.0000000000000000 | 1.56pg_catalog | pg_rewrite | 120 kB | 157.8666666666666667 | 1.93pg_catalog | pg_trigger | 8192 bytes | 0.00000000000000000000 | 0.00pg_catalog | pg_description | 344 kB | 97.4883720930232558 | 1.19pg_catalog | pg_cast | 16 kB | 2208.0000000000000000 | 26.95pg_catalog | pg_namespace | 8192 bytes | 7584.0000000000000000 | 92.58pg_catalog | pg_conversion | 16 kB | 224.0000000000000000 | 2.73pg_catalog | pg_depend | 128 kB | 0.00000000000000000000 | 0.00pg_catalog | pg_database | 8192 bytes | 7008.0000000000000000 | 85.55pg_catalog | pg_tablespace | 8192 bytes | 7936.0000000000000000 | 96.88pg_catalog | pg_auth_members | 8192 bytes | 8000.0000000000000000 | 97.66pg_catalog | pg_shdescription | 8192 bytes | 7936.0000000000000000 | 96.88pg_catalog | pg_ts_config | 8192 bytes | 5024.0000000000000000 | 61.33pg_catalog | pg_ts_config_map | 24 kB | 64.0000000000000000 | 0.78pg_catalog | pg_ts_dict | 8192 bytes | 4000.0000000000000000 | 48.83pg_catalog | pg_ts_parser | 8192 bytes | 8032.0000000000000000 | 98.05pg_catalog | pg_ts_template | 8192 bytes | 7616.0000000000000000 | 92.97pg_catalog | pg_extension | 8192 bytes | 8032.0000000000000000 | 98.05pg_catalog | pg_init_privs | 32 kB | 16.0000000000000000 | 0.20pg_catalog | pg_collation | 224 kB | 265.1428571428571429 | 3.24pg_catalog | pg_range | 8192 bytes | 7776.0000000000000000 | 94.92pg_catalog | pg_sequence | 8192 bytes | 0.00000000000000000000 | 0.00information_schema | sql_features | 64 kB | 704.0000000000000000 | 8.59information_schema | sql_implementation_info | 8192 bytes | 7296.0000000000000000 | 89.06information_schema | sql_parts | 8192 bytes | 7328.0000000000000000 | 89.45information_schema | sql_sizing | 8192 bytes | 6176.0000000000000000 | 75.39
场景2：自动VACUUM决策
-- 找出需要VACUUM的表（空闲空间超过30%）
SELECT n.nspname AS schema,c.relname AS table,round((SELECT sum(avail) FROM pg_freespace(c.oid)) * 100.0 / NULLIF(pg_relation_size(c.oid), 0), 2) AS free_percent
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind = 'r'
AND (SELECT sum(avail) FROM pg_freespace(c.oid)) > 0.3 * pg_relation_size(c.oid)
ORDER BY free_percent DESC;
输出示例：
white=# SELECT
white-# n.nspname AS schema,
white-# c.relname AS table,
white-# round((SELECT sum(avail) FROM pg_freespace(c.oid)) * 100.0 /
white(# NULLIF(pg_relation_size(c.oid), 0), 2) AS free_percent
white-# FROM pg_class c
white-# JOIN pg_namespace n ON n.oid = c.relnamespace
white-# WHERE c.relkind = 'r'
white-# AND (SELECT sum(avail) FROM pg_freespace(c.oid)) >
white-# 0.3 * pg_relation_size(c.oid)
white-# ORDER BY free_percent DESC;schema | table | free_percent
--------------------+-------------------------+--------------pg_catalog | pg_ts_parser | 98.05pg_catalog | pg_extension | 98.05pg_catalog | pg_auth_members | 97.66pg_catalog | pg_shdescription | 96.88pg_catalog | pg_tablespace | 96.88pg_catalog | pg_range | 94.92pg_catalog | pg_language | 93.36pg_catalog | pg_ts_template | 92.97pg_catalog | pg_namespace | 92.58pg_catalog | pg_am | 90.23information_schema | sql_parts | 89.45information_schema | sql_implementation_info | 89.06pg_catalog | pg_database | 85.55information_schema | sql_sizing | 75.39pg_catalog | pg_authid | 73.05pg_catalog | pg_ts_config | 61.33pg_catalog | pg_ts_dict | 48.83
(17 rows)white=#
七 注意事项
- 性能影响：频繁查询FSM会对系统性能产生一定影响，建议在非高峰期使用
- 权限要求：需要超级用户或表所有者权限
- 实时性：FSM信息不是实时更新的，VACUUM操作后会更新
- 外部表：不适用于外部表
- TOAST表：需要单独检查TOAST表的空闲空间
八 与VACUUM的关系
- VACUUM会更新FSM信息
- FSM大小由参数
max_fsm_pages和max_fsm_relations控制 - 可以使用
VACUUM VERBOSE查看FSM更新情况
-- 调整FSM参数（需要重启）
ALTER SYSTEM SET max_fsm_pages = 200000;
ALTER SYSTEM SET max_fsm_relations = 10000;
九 故障排查
问题1：扩展无法创建
解决方案：
-- 检查是否在正确数据库创建
SELECT current_database();-- 检查扩展是否已存在
SELECT * FROM pg_available_extensions WHERE name = 'pg_freespacemap';-- 以超级用户身份创建
\c - postgres
CREATE EXTENSION pg_freespacemap;
问题2：查询返回空结果
可能原因：
- 表太小（小于1个块）
- 没有空闲空间
- 权限不足
验证方法：
-- 检查表大小
SELECT pg_size_pretty(pg_relation_size('table_name'));-- 检查权限
\z table_name
通过合理使用pg_freespacemap扩展，数据库管理员可以有效监控和管理PostgreSQL表的空间使用情况，优化存储效率并减少不必要的空间浪费。