PostgreSQL 数据库大小大于预期

Question

我有下一个 psql 数据库：

CREATE TABLE "readings33" (
    "uniqueid" BIGSERIAL PRIMARY KEY,
    "uniqueid_sensor" INTEGER NOT NULL,
    "timestamp" TIMESTAMP NOT NULL DEFAULT NULL,
    "value" VARCHAR(15) NOT NULL,
    CONSTRAINT "FK_readings_sensors" FOREIGN KEY ("uniqueid_sensor") REFERENCES "public"."sensors" ("uniqueid") ON UPDATE NO ACTION ON DELETE CASCADE   
);

AFAIK 总大小应该是：

"uniqueid" -> 8 bytes
"uniqueid_sensor" -> 4 bytes
"timestamp" -> 10 bytes
"value" VARCHAR(15) 8 bytes (because my value length for the test is a string with 8 bytes)

所有的总和是 8+4+10+8 = 30 字节，但是当我将 100.000 行写入数据库时​​，这占用了 12.5 Mib，即每行 125 字节。我已经用 10.000 行完成了这个文本，并且关系大致相同......谁能告诉我为什么这个增量是大小？？

提前致谢

Answer 1

最后在运行this脚本后发现大小是正确的，不知道HeidiSQL为什么给我一个错误的大小：

WITH cteTableInfo AS 
(
    SELECT 
        COUNT(1) AS ct
        ,SUM(length(t::text)) AS TextLength  
        ,'public.readings'::regclass AS TableName  
    FROM public.readings AS t  
)
,cteRowSize AS 
(
   SELECT ARRAY [pg_relation_size(TableName)
               , pg_relation_size(TableName, 'vm')
               , pg_relation_size(TableName, 'fsm')
               , pg_table_size(TableName)
               , pg_indexes_size(TableName)
               , pg_total_relation_size(TableName)
               , TextLength
             ] AS val
        , ARRAY ['Total Relation Size'
               , 'Visibility Map'
               , 'Free Space Map'
               , 'Table Included Toast Size'
               , 'Indexes Size'
               , 'Total Toast and Indexes Size'
               , 'Live Row Byte Size'
             ] AS Name
   FROM cteTableInfo
)
SELECT 
    unnest(name) AS Description
    ,unnest(val) AS Bytes
    ,pg_size_pretty(unnest(val)) AS BytesPretty
    ,unnest(val) / ct AS bytes_per_row
FROM cteTableInfo, cteRowSize
 
UNION ALL SELECT '------------------------------', NULL, NULL, NULL
UNION ALL SELECT 'TotalRows', ct, NULL, NULL FROM cteTableInfo
UNION ALL SELECT 'LiveTuples', pg_stat_get_live_tuples(TableName), NULL, NULL FROM cteTableInfo
UNION ALL SELECT 'DeadTuples', pg_stat_get_dead_tuples(TableName), NULL, NULL FROM cteTableInfo;

结果：

Answer 2

一个timestamp占用8个字节，但是integer后面会有4个字节的填充。一个 8 字节的字符串将占用 9 个字节。

所以行数据占用 33 个字节。连同 23 字节的行标题和大约 60 字节的附加填充。最后，每个 8kB 数据块有一些开销。

如果你得到的远不止这些，那么要么你的测量结果是错误的（你是否包含了主键索引？），或者你的表因数据修改或非标准的fillfactor 设置而变得臃肿。

要测量表格的实际大小，请使用

SELECT pg_table_size('readings33');

要测量包括索引在内的大小，请使用

SELECT pg_total_relation_size('readings33');