前言
关系膨胀概述
VACUUM不能解决表膨胀的问题，死元组本身能够被并发
VACUUM机制回收，但它产生的碎片，留下的空洞却不可以。比如，即使删除了许多死元组，也无法减小表的大小。久而久之，关系文件被大量空洞填满，浪费了大量的磁盘空间。VACUUM FULL
命令可以回收这些空间，它将旧表文件中的活元组复制到新表中，通过重写整张表的方式将表压实。但在实际生产中，因为该操作会持有表上的
AccessExclusiveLock，阻塞业务正常访问，因此在不间断服务的情况下并不适用，
pg_repack是一个实用的第三方插件，能够在线上业务正常进行的同时进行无锁的
VACUUM FULL。VACUUM FULL处理膨胀并没有一个最佳实践。DBA需要针对自己的业务场景制定清理策略。但无论采用何种策略，实施这些策略的机制都是类似的：关系膨胀的度量
膨胀率的精确计算
pgstattuple模块，可用于精确计算表的膨胀率。譬如这里的
tuple_percent字段就是元组实际字节占关系总大小的百分比，用1减去该值即为膨胀率。select *, 1.0 - tuple_len::numeric / table_len as bloat
from pgstattuple('pgbench_accounts');
│ table_len │ 136642560 │
│ tuple_count │ 1000000 │
│ tuple_len │ 121000000 │
│ tuple_percent │ 88.55 │
│ dead_tuple_count │ 16418 │
│ dead_tuple_len │ 1986578 │
│ dead_tuple_percent │ 1.45 │
│ free_space │ 1674768 │
│ free_percent │ 1.23 │
│ bloat │ 0.11447794889088729017 │
pgstattuple对于精确地判断表与索引的膨胀情况非常有用，具体细节可以参考官方文档：
https://www.postgresql.org/docs/current/static/pgstattuple.html
。pg_freespacemap与
pageinspect，前者可以用于检视每个页面中的空闲空间大小，后者则可以精确地展示关系中每个数据页内物理存储的内容。如果希望检视关系的内部状态，这两个插件非常实用，详细使用方法可以参考官方文档：pgstattuple插件执行精确的膨胀率估算大致需要5分钟时间。在9.5及后续版本，
pgstattuple插件还提供了
pgstattuple_approx函数，以精度换速度。但即使使用估算，也需要秒级的时间。膨胀率的估算
avgwidth：从列级统计数据计算而来，用于估计紧实状态占用的空间。pg_class.reltuples：用于估计紧实状态占用的空间pg_class.relpages：用于测算实际使用的空间1 - (reltuples * avgwidth) / (block_size - pageheader) / relpages
block_size是页面大小，默认为8182，
pageheader是首部占用的大小，默认为24字节。页面大小减去首部大小就是可以用于元组存储的实际空间，因此
(reltuples * avgwidth)给出了元组的估计总大小，而除以前者后，就可以得到预计需要多少个页面才能紧实地存下所有的元组。最后，期待使用的页面数量，除以实际使用的页面数量，就是
利用率，而1减去利用率，就是膨胀率。难点
计算元组的平均长度
MAXALIGN，通常为8。WITH OIDS选项，元组还会有一个4字节的OID，但这里我们不考虑该情况。avg_size_tuple = 4 + avg_size_hdr + avg_size_data
计算首部的平均长度
normhdrnullhdrnullfracavg_size_hdr = nullhdr * nullfrac + normhdr * (1 - nullfrac)
avg_size_hdr = nullhdr * nullfrac + 24 * (1 - nullfrac)
padding = lambda x : x + 7 >> 3 << 3
计算数据部分的平均长度
SELECT schemaname, tablename, sum((1 - null_frac) * avg_width) FROM pg_stats GROUP BY (schemaname, tablename);
pg_stats系统统计视图中获取
app.apple表上一条元组的平均长度。SELECT count(*), -- 字段数目 ceil(count(*) / 8.0), -- 空值位图占用的字节数 max(null_frac), -- 最大空值率 sum((1 - null_frac) * avg_width) -- 数据部分的平均宽度 FROM pg_stats where schemaname = 'app' and tablename = 'apple'; -[ RECORD 1 ]----------- count | 47 ceil | 6 max | 1 sum | 1733.76873471724
整合
CREATE OR REPLACE FUNCTION public . pg_table_bloat (relation regclass )
RETURNS double precision LANGUAGE plpgsql AS $function$ DECLARE _schemaname text; tuples BIGINT := 0; pages INTEGER := 0; nullheader INTEGER:= 0; nullfrac FLOAT := 0; datawidth INTEGER :=0; avgtuplelen FLOAT :=24; BEGIN SELECT relnamespace :: RegNamespace, reltuples, relpages into _schemaname, tuples, pages FROM pg_class Where oid = relation; SELECT 23 + ceil(count(*) >> 3), max(null_frac), ceil(sum((1 - null_frac) * avg_width)) into nullheader, nullfrac, datawidth FROM pg_stats where schemaname = _schemaname and tablename = relation :: text; SELECT (datawidth + 8 - (CASE WHEN datawidth%8=0 THEN 8 ELSE datawidth%8 END)) -- avg data len + (1 - nullfrac) * 24 + nullfrac * (nullheader + 8 - (CASE WHEN nullheader%8=0 THEN 8 ELSE nullheader%8 END)) INTO avgtuplelen; raise notice '% %', nullfrac, datawidth; RETURN 1 - (ceil(tuples * avgtuplelen / 8168)) / pages; END; $function$
批量计算
DROP VIEW IF EXISTS monitor.pg_bloat_indexes CASCADE;
CREATE OR REPLACE VIEW monitor.pg_bloat_indexes AS
WITH btree_index_atts AS (
SELECT
pg_namespace.nspname,
indexclass.relname AS index_name,
indexclass.reltuples,
indexclass.relpages,
pg_index.indrelid,
pg_index.indexrelid,
indexclass.relam,
tableclass.relname AS tablename,
(regexp_split_to_table((pg_index.indkey) :: TEXT, ' ' :: TEXT)) :: SMALLINT AS attnum,
pg_index.indexrelid AS index_oid
FROM ((((pg_index
JOIN pg_class indexclass ON ((pg_index.indexrelid = indexclass.oid)))
JOIN pg_class tableclass ON ((pg_index.indrelid = tableclass.oid)))
JOIN pg_namespace ON ((pg_namespace.oid = indexclass.relnamespace)))
JOIN pg_am ON ((indexclass.relam = pg_am.oid)))
WHERE ((pg_am.amname = 'btree' :: NAME) AND (indexclass.relpages > 0))
), index_item_sizes AS (
SELECT
ind_atts.nspname,
ind_atts.index_name,
ind_atts.reltuples,
ind_atts.relpages,
ind_atts.relam,
ind_atts.indrelid AS table_oid,
ind_atts.index_oid,
(current_setting('block_size' :: TEXT)) :: NUMERIC AS bs,
8 AS maxalign,
24 AS pagehdr,
CASE
WHEN (max(COALESCE(pg_stats.null_frac, (0) :: REAL)) = (0) :: FLOAT)
THEN 2
ELSE 6
END AS index_tuple_hdr,
sum((((1) :: FLOAT - COALESCE(pg_stats.null_frac, (0) :: REAL)) *
(COALESCE(pg_stats.avg_width, 1024)) :: FLOAT)) AS nulldatawidth
FROM ((pg_attribute
JOIN btree_index_atts ind_atts
ON (((pg_attribute.attrelid = ind_atts.indexrelid) AND (pg_attribute.attnum = ind_atts.attnum))))
JOIN pg_stats ON (((pg_stats.schemaname = ind_atts.nspname) AND (((pg_stats.tablename = ind_atts.tablename) AND
((pg_stats.attname) :: TEXT =
pg_get_indexdef(pg_attribute.attrelid,
(pg_attribute.attnum) :: INTEGER,
TRUE))) OR
((pg_stats.tablename = ind_atts.index_name) AND
(pg_stats.attname = pg_attribute.attname))))))
WHERE (pg_attribute.attnum > 0)
GROUP BY ind_atts.nspname, ind_atts.index_name, ind_atts.reltuples, ind_atts.relpages, ind_atts.relam,
ind_atts.indrelid, ind_atts.index_oid, (current_setting('block_size' :: TEXT)) :: NUMERIC, 8 :: INTEGER
), index_aligned_est AS (
SELECT
index_item_sizes.maxalign,
index_item_sizes.bs,
index_item_sizes.nspname,
index_item_sizes.index_name,
index_item_sizes.reltuples,
index_item_sizes.relpages,
index_item_sizes.relam,
index_item_sizes.table_oid,
index_item_sizes.index_oid,
COALESCE(ceil((((index_item_sizes.reltuples * ((((((((6 + index_item_sizes.maxalign) -
CASE
WHEN ((index_item_sizes.index_tuple_hdr %
index_item_sizes.maxalign) = 0)
THEN index_item_sizes.maxalign
ELSE (index_item_sizes.index_tuple_hdr %
index_item_sizes.maxalign)
END)) :: FLOAT + index_item_sizes.nulldatawidth)
+ (index_item_sizes.maxalign) :: FLOAT) - (
CASE
WHEN (((index_item_sizes.nulldatawidth) :: INTEGER %
index_item_sizes.maxalign) = 0)
THEN index_item_sizes.maxalign
ELSE ((index_item_sizes.nulldatawidth) :: INTEGER %
index_item_sizes.maxalign)
END) :: FLOAT)) :: NUMERIC) :: FLOAT) /
((index_item_sizes.bs - (index_item_sizes.pagehdr) :: NUMERIC)) :: FLOAT) +
(1) :: FLOAT)), (0) :: FLOAT) AS expected
FROM index_item_sizes
), raw_bloat AS (
SELECT
current_database() AS dbname,
index_aligned_est.nspname,
pg_class.relname AS table_name,
index_aligned_est.index_name,
(index_aligned_est.bs * ((index_aligned_est.relpages) :: BIGINT) :: NUMERIC) AS totalbytes,
index_aligned_est.expected,
CASE
WHEN ((index_aligned_est.relpages) :: FLOAT <= index_aligned_est.expected)
THEN (0) :: NUMERIC
ELSE (index_aligned_est.bs *
((((index_aligned_est.relpages) :: FLOAT - index_aligned_est.expected)) :: BIGINT) :: NUMERIC)
END AS wastedbytes,
CASE
WHEN ((index_aligned_est.relpages) :: FLOAT <= index_aligned_est.expected)
THEN (0) :: NUMERIC
ELSE (((index_aligned_est.bs * ((((index_aligned_est.relpages) :: FLOAT -
index_aligned_est.expected)) :: BIGINT) :: NUMERIC) * (100) :: NUMERIC) /
(index_aligned_est.bs * ((index_aligned_est.relpages) :: BIGINT) :: NUMERIC))
END AS realbloat,
pg_relation_size((index_aligned_est.table_oid) :: REGCLASS) AS table_bytes,
stat.idx_scan AS index_scans
FROM ((index_aligned_est
JOIN pg_class ON ((pg_class.oid = index_aligned_est.table_oid)))
JOIN pg_stat_user_indexes stat ON ((index_aligned_est.index_oid = stat.indexrelid)))
), format_bloat AS (
SELECT
raw_bloat.dbname AS database_name,
raw_bloat.nspname AS schema_name,
raw_bloat.table_name,
raw_bloat.index_name,
round(
raw_bloat.realbloat) AS bloat_pct,
round((raw_bloat.wastedbytes / (((1024) :: FLOAT ^
(2) :: FLOAT)) :: NUMERIC)) AS bloat_mb,
round((raw_bloat.totalbytes / (((1024) :: FLOAT ^ (2) :: FLOAT)) :: NUMERIC),
3) AS index_mb,
round(
((raw_bloat.table_bytes) :: NUMERIC / (((1024) :: FLOAT ^ (2) :: FLOAT)) :: NUMERIC),
3) AS table_mb,
raw_bloat.index_scans
FROM raw_bloat
)
SELECT
format_bloat.database_name as datname,
format_bloat.schema_name as nspname,
format_bloat.table_name as relname,
format_bloat.index_name as idxname,
format_bloat.index_scans as idx_scans,
format_bloat.bloat_pct as bloat_pct,
format_bloat.table_mb,
format_bloat.index_mb - format_bloat.bloat_mb as actual_mb,
format_bloat.bloat_mb,
format_bloat.index_mb as total_mb
FROM format_bloat
ORDER BY format_bloat.bloat_mb DESC;
COMMENT ON VIEW monitor.pg_bloat_indexes IS 'index bloat monitor';
表膨胀的处理
VACUUM FULL就可以了。但对于需要不间断运行的数据库，我们就需要用到
pg_repack来处理表的膨胀。pg_repack已经包含在了PostgreSQL官方的yum源中，因此可以直接通过
yum install pg_repack安装。yum install pg_repack10
pg_repack的使用
pg_repack也通过类似的参数连接至PostgreSQL服务器。pg_repack之前，需要在待重整的数据库中创建
pg_repack扩展CREATE EXTENSION pg_repack
# 完全清理整个数据库，开5个并发任务，超时等待10秒 pg_repack -d <database> -j 5 -T 10 # 清理mydb中一张特定的表mytable，超时等待10秒 pg_repack mydb -t public.mytable -T 10 # 清理某个特定的索引 myschema.myindex，注意必须使用带模式的全名 pg_repack mydb -i myschema.myindex
pg_repack的策略
#--------------------------------------------------------------#
# Name: repack_tables
# Desc: repack table via fullname
# Arg1: database_name
# Argv: list of table full name
# Deps: psql
#--------------------------------------------------------------#
# repack single table
function repack_tables(){
local db=$1
shift
log_info "repack ${db} tables begin"
log_info "repack table list: $@"
for relname in $@
do
old_size=$(psql ${db} -Atqc "SELECT pg_size_pretty(pg_relation_size('${relname}'));")
# kill_queries ${db}
log_info "repack table ${relname} begin, old size: ${old_size}"
pg_repack ${db} -T 10 -t ${relname}
new_size=$(psql ${db} -Atqc "SELECT pg_size_pretty(pg_relation_size('${relname}'));")
log_info "repack table ${relname} done , new size: ${old_size} -> ${new_size}"
done
log_info "repack ${db} tables done"
}
#--------------------------------------------------------------#
# Name: get_bloat_tables
# Desc: find bloat tables in given database match some condition
# Arg1: database_name
# Echo: list of full table name
# Deps: psql, monitor.pg_bloat_tables
#--------------------------------------------------------------#
function get_bloat_tables(){
echo $(psql ${1} -Atq <<-'EOF'
WITH bloat_tables AS (
SELECT
nspname || '.' || relname as relname,
actual_mb,
bloat_pct
FROM monitor.pg_bloat_tables
WHERE nspname NOT IN ('dba', 'monitor', 'trash')
ORDER BY 2 DESC,3 DESC
)
-- 64 small + 16 medium + 4 large
(SELECT relname FROM bloat_tables WHERE actual_mb < 256 AND bloat_pct > 40 ORDER BY bloat_pct DESC LIMIT 64) UNION
(SELECT relname FROM bloat_tables WHERE actual_mb BETWEEN 256 AND 1024 AND bloat_pct > 30 ORDER BY bloat_pct DESC LIMIT 16) UNION
(SELECT relname FROM bloat_tables WHERE actual_mb BETWEEN 1024 AND 4096 AND bloat_pct > 20 ORDER BY bloat_pct DESC LIMIT 4);
EOF
)
}
pg_repack的原理
pg_repack的原理相当简单，它会为待重建的表创建一份副本。首先取一份全量快照，将所有活元组写入新表，并通过触发器将所有针对原表的变更同步至新表，最后通过重命名，使用新的紧实副本替换老表。而对于索引，则是通过PostgreSQL的
CREATE(DROP) INDEX CONCURRENTLY完成的。-
创建一张原始表的相应日志表。 -
为原始表添加行触发器，在相应日志表中记录所有 INSERT,DELETE,UPDATE操作。 -
创建一张包含老表所有行的表。 -
在新表上创建同样的索引 -
将日志表中的增量变更应用到新表上 -
使用系统目录切换表，相关索引，相关Toast表。
-
使用 CREATE INDEX CONCURRENTLY在原表上创建新索引，保持与旧索引相同的定义。 -
在数据目录中将新旧索引交换。 -
删除旧索引。
pg_repack的注意事项
${schema_name}.table_${table_oid}${schema_name}.index_${table_oid}}文章来源于非法加冯 ，作者Vonng
