限制子句在 postgres 中工作？

Question

所以我有一个有1000行的表，我只是说

select * from hugedata limit 1

查询计划只是说

table scan

然后

limit operator

附加的是查询计划

    [
  {
    "Plan": {
      "Node Type": "Limit",
      "Parallel Aware": false,
      "Async Capable": false,
      "Startup Cost": 0.00,
      "Total Cost": 0.02,
      "Plan Rows": 1,
      "Plan Width": 41,
      "Actual Startup Time": 0.007,
      "Actual Total Time": 0.007,
      "Actual Rows": 1,
      "Actual Loops": 1,
      "Output": ["pk", "description", "flags"],
      "Shared Hit Blocks": 1,
      "Shared Read Blocks": 0,
      "Shared Dirtied Blocks": 0,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0,
      "WAL Records": 0,
      "WAL FPI": 0,
      "WAL Bytes": 0,
      "Plans": [
        {
          "Node Type": "Seq Scan",
          "Parent Relationship": "Outer",
          "Parallel Aware": false,
          "Async Capable": false,
          "Relation Name": "hugedata",
          "Schema": "public",
          "Alias": "hugedata",
          "Startup Cost": 0.00,
          "Total Cost": 15406.01,
          "Plan Rows": 1000001,
          "Plan Width": 41,
          "Actual Startup Time": 0.006,
          "Actual Total Time": 0.006,
          "Actual Rows": 1,
          "Actual Loops": 1,
          "Output": ["pk", "description", "flags"],
          "Shared Hit Blocks": 1,
          "Shared Read Blocks": 0,
          "Shared Dirtied Blocks": 0,
          "Shared Written Blocks": 0,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 0,
          "Temp Written Blocks": 0,
          "WAL Records": 0,
          "WAL FPI": 0,
          "WAL Bytes": 0
        }
      ]
    },
    "Settings": {
    },
    "Planning": {
      "Shared Hit Blocks": 0,
      "Shared Read Blocks": 0,
      "Shared Dirtied Blocks": 0,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0
    },
    "Planning Time": 0.035,
    "Triggers": [
    ],
    "Execution Time": 0.069
  }
]

问题是限制子句在postgres中是如何应用的？

1. 数据库引擎选择 1000 条（全部）记录，然后将这些记录传递给 limit 运算符，它接受第一个记录并丢弃 999 条记录..

或

2. 数据库引擎使用接受它的限制运算符检索第一个记录检查，然后数据库引擎尝试使用拒绝它的限制运算符搜索第二个记录检查，并且数据库引擎停止进一步搜索...与第一个记录的区别是数据库引擎始终适用于（搜索/检索）N+1（限制数量）行，其中在第一个行中它总是获取表中的所有行..

从我的搜索中，它告诉我限制运算符的工作原理与第一点建议的一样，如果这是真的，为什么实现效率如此低下，或者我错过了什么？

Limit 文档说它在生成查询计划时考虑了限制，但它是否在 N+1 行产生产量或考虑意味着未指定其他内容...https://www.postgresql.org/docs/current /queries-limit.html

Answer 1

PostgreSQL 执行器按需自上而下工作。

为了计算第一个结果行，它从

Limit

节点获取第一行。依次从

Seq Scan

中获取第一行。

当 PostgreSQL 从

Limit

节点获取第二个结果行时，通知执行完成。

总之，从顺序扫描中仅获取一行。