{"Entry":{"collection":"plan","key":"seq-scan","name":"Seq Scan","aliases":["Seq Scan","SeqScan","T_SeqScan"],"metadata":{"aliases":["Seq Scan","SeqScan","T_SeqScan"],"category":"Scan","content_hash":"9ab82aa4b6c2f0aa7a079f4847bc7b25fb25745e8e050604f944c7496f166391","derived_content_hash":"12a06ee9cd295704db9c58716db0bca981b16af89fe7a2d9900da0a9f3a663ab","imported_at":"2026-09-30T00:40:44.246834+08:00","name":"Seq Scan","name_zh":"SeqScan","slug":"seq-scan","summary":"Reads a relation through a sequential scan and applies its scan qualifications."}},"Definition":{"Collection":"plan","Key":"seq-scan","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"seq-scan","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitSeqScan","memory_mechanism":"unclassified","node_tag":"T_SeqScan","parallel_callbacks":["ExecSeqScanEstimate","ExecSeqScanInitializeDSM","ExecSeqScanInitializeWorker","ExecSeqScanReInitializeDSM"],"partial_modes":[],"strategies":[],"text_names":["Seq Scan"]},"comparison_hash":"5631c0db91382334dc4ca321ae0335013d10ae1c0c24f3523fcca0184bbbc81b","description":["Reads a relation through a sequential scan and applies its scan qualifications."],"description_zh":"顺序扫描关系并应用扫描条件。","description_zh_html":"\u003cp\u003e顺序扫描关系并应用扫描条件。\u003c/p\u003e\u003cp\u003e核心节点标识：\u003ccode\u003eT_SeqScan\u003c/code\u003e；EXPLAIN 名称：\u003ccode\u003eSeq Scan\u003c/code\u003e。\u003c/p\u003e\u003cp\u003e本版本源码中的并行执行或观测回调：\u003ccode\u003eExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM\u003c/code\u003e。回调的存在不表示该节点必然支持共享并行扫描；在并行工作进程中执行也不等于具有并行感知能力。\u003c/p\u003e\u003ch3\u003e同版本中文手册节选\u003c/h3\u003e\u003cp\u003e\n    查询计划的结构是一棵由\u003cem class=\"firstterm\"\u003e计划节点\u003c/em\u003e组成的树。树的最底层是扫描节点，它们从表中返回原始行。不同的表访问方法对应不同类型的扫描节点，例如顺序扫描、索引扫描和位图索引扫描。也有一些并非表的行来源，例如 \u003ccode class=\"literal\"\u003eVALUES\u003c/code\u003e 子句和 \u003ccode class=\"literal\"\u003eFROM\u003c/code\u003e 中的返回集合函数，它们也各自有对应的扫描节点类型。如果查询需要对原始行执行连接、聚合、排序或其他操作，那么扫描节点之上还会出现额外的节点来完成这些操作。同样，这些操作通常不止一种实现方式，因此这里也会出现不同的节点类型。\u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e 会为计划树中的每个节点输出一行，显示基本节点类型以及规划器对该计划节点执行代价的估计值。还可能出现相对节点摘要行缩进的附加行，用来显示该节点的更多属性。第一行，也就是最顶层节点的摘要行，给出了整个计划的估计总执行代价；规划器力图最小化的正是这个数字。\n   \u003c/p\u003e\u003cp\u003e\n    下面是一个简单的示例，仅用于展示输出的样子：\n\n\u003c/p\u003e\u003cp\u003e\n    由于这个查询没有 \u003ccode class=\"literal\"\u003eWHERE\u003c/code\u003e 子句，它必须扫描表中的所有行，因此规划器选择了一个简单的顺序扫描计划。圆括号中的数字从左到右依次表示：\n\n    \u003c/p\u003e\u003cp\u003e\n    这些代价使用由规划器代价参数决定的任意单位来衡量（见\u003ca class=\"xref\" href=\"/zh/docs/18/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS\"\u003e第 19.7.2 节\u003c/a\u003e）。传统上通常以磁盘页读取作为代价单位；也就是说，惯例上将 \u003ca class=\"xref\" href=\"/zh/docs/18/runtime-config-query.html#GUC-SEQ-PAGE-COST\"\u003eseq_page_cost\u003c/a\u003e 设为 \u003ccode class=\"literal\"\u003e1.0\u003c/code\u003e，其他代价参数都相对它来设定。本节中的示例都使用默认代价参数。\n   \u003c/p\u003e\u003cp\u003e\u003ca href=\"/zh/docs/18/using-explain.html#USING-EXPLAIN-BASICS\"\u003e查看完整中文手册与上下文\u003c/a\u003e。\u003c/p\u003e\u003cp\u003e本页开头的中文说明为 PG.CENTER 对编辑说明的翻译；手册节选保留对应版本中文文档原文。源码信息及手册示例不代表重新进行过运行测试。\u003c/p\u003e","evidence_kind":"source and documentation","explain_names":["Seq Scan"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_SeqScan"},{"label":"Structured EXPLAIN Node Type","value":"Seq Scan"},{"label":"Inputs","value":"A relation"},{"label":"Output","value":"Qualified relation tuples"},{"label":"Executor initializer","value":"ExecInitSeqScan"},{"label":"Memory mechanism","value":"unclassified"}],"facts_zh":[{"label":"核心节点标签","value":"T_SeqScan"},{"label":"结构化 EXPLAIN 节点类型","value":"Seq Scan"},{"label":"输入","value":"关系"},{"label":"输出","value":"满足条件的关系元组"},{"label":"执行器初始化函数","value":"ExecInitSeqScan"},{"label":"内存机制","value":"unclassified"}],"memory":{"description":"This extraction does not assign a universal memory limit or spill policy to this node. 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Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Seq Scan.","Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node."],"title":"EXPLAIN names and attributes"},{"paragraphs":["This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider."],"title":"Memory and temporary storage"},{"paragraphs":["The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.","Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."],"title":"Parallel execution and instrumentation"},{"paragraphs":["Notice that the EXPLAIN output shows the WHERE clause being applied as a “ filter ” condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition."],"title":"Same-version manual discussion"},{"blocks":[{"code":"EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","Here is a trivial example, just to show what the output looks like:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN SELECT * FROM tenk1 WHERE unique1 \u003c 7000;\n\n                         QUERY 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a scan that was planned to be parallel.","SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual","Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not."],"title":"Executor implementation notes"},{"code":"case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;","title":"EXPLAIN identity in core source"}],"sections_zh":[{"paragraphs":["结构化格式使用上述 Node Type。文本格式名称还可能包含操作、策略、连接类型、扫描方向或聚合阶段属性。","此源码记录的文本名称：Seq Scan.","并行感知与并行安全是不同的计划属性。在并行工作进程内运行的节点不一定是并行感知节点。"],"title":"EXPLAIN 名称与属性"},{"paragraphs":["本次抽取不为此节点设定统一的内存上限或落盘策略。请查看同一构建的实现、相关表达式或提供方。"],"title":"内存与临时存储"},{"paragraphs":["以下源码回调可以协调执行或收集工作进程的测量数据。回调存在不代表该节点普遍支持共享并行扫描或共享状态。","此构建的回调：ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."],"title":"并行执行与运行信息采集"},{"paragraphs":["EXPLAIN 将 WHERE 子句显示为 Seq Scan 节点上的 filter 条件。这表示节点对扫描的每一行检查条件，仅输出通过条件的行。WHERE 降低了预计输出行数，但扫描仍需访问全部 10000 行，所以成本没有降低；相反还略有增加（准确说增加 10000 * cpu_operator_cost），以计入检查 WHERE 条件消耗的额外 CPU 时间。"],"title":"同版本手册说明"},{"blocks":[{"code":"EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)","paragraphs":["示例摘自 PostgreSQL 18.6 手册；本百科未实际执行此示例。","下面是一个简单示例，仅用来展示输出形式："],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN SELECT * FROM tenk1 WHERE unique1 \u003c 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 \u003c 7000)","paragraphs":["示例摘自 PostgreSQL 18.6 手册；本百科未实际执行此示例。","现在修改查询，添加 WHERE 条件："],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}}],"title":"本版手册中的示例"},{"paragraphs":["nodeSeqscan.c：支持关系顺序扫描的例程。","ExecSeqScanEstimate 估算并行扫描所需的 DSM 空间；ExecSeqScanInitializeDSM 初始化并行扫描的 DSM；ExecSeqScanReInitializeDSM 为新一轮并行扫描重新初始化 DSM；ExecSeqScanInitializeWorker 在并行工作进程中附加到 DSM 信息。","扫描不并行执行，或原计划并行的扫描以串行方式执行时，会到达此处。","SeqRecheck：在 EvalPlanQual 中重新检查元组的访问方法例程。","与 IndexScan 不同，SeqScan 从不在 heap_beginscan 中使用键（这很不好），因此这里不检查键是否满足条件。"],"title":"执行器实现说明"},{"code":"case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;","title":"核心源码中的 EXPLAIN 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Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Seq Scan.","Parallel-aware and parallel-safe are different plan properties. A node running inside a parallel worker is not necessarily a parallel-aware node."],"title":"EXPLAIN names and attributes"},{"paragraphs":["This extraction does not assign a universal memory limit or spill policy to this node. Inspect the same-build implementation and its expressions or provider."],"title":"Memory and temporary storage"},{"paragraphs":["The source callbacks below can coordinate execution or collect worker instrumentation. Their presence is not a blanket claim that this node supports a shared parallel scan or shared state.","Callbacks in this build: ExecSeqScanEstimate, ExecSeqScanInitializeDSM, ExecSeqScanInitializeWorker, ExecSeqScanReInitializeDSM."],"title":"Parallel execution and instrumentation"},{"paragraphs":["Notice that the EXPLAIN output shows the WHERE clause being applied as a “ filter ” condition attached to the Seq Scan plan node. This means that the plan node checks the condition for each row it scans, and outputs only the ones that pass the condition. The estimate of output rows has been reduced because of the WHERE clause. However, the scan will still have to visit all 10000 rows, so the cost hasn't decreased; in fact it has gone up a bit (by 10000 * cpu_operator_cost , to be exact) to reflect the extra CPU time spent checking the WHERE condition."],"title":"Same-version manual discussion"},{"blocks":[{"code":"EXPLAIN SELECT * FROM tenk1;\n\n                         QUERY PLAN\n-------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..445.00 rows=10000 width=244)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","Here is a trivial example, just to show what the output looks like:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN SELECT * FROM tenk1 WHERE unique1 \u003c 7000;\n\n                         QUERY PLAN\n------------------------------------------------------------\n Seq Scan on tenk1  (cost=0.00..470.00 rows=7000 width=244)\n   Filter: (unique1 \u003c 7000)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","Now let's modify the query to add a WHERE condition:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}}],"title":"Examples from this manual build"},{"paragraphs":["nodeSeqscan.c Support routines for sequential scans of relations.","ExecSeqScanEstimate estimates DSM space needed for parallel scan ExecSeqScanInitializeDSM initialize DSM for parallel scan ExecSeqScanReInitializeDSM reinitialize DSM for fresh parallel scan ExecSeqScanInitializeWorker attach to DSM info in parallel worker","We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel.","SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual","Note that unlike IndexScan, SeqScan never use keys in heap_beginscan (and this is very bad) - so, here we do not check are keys ok or not."],"title":"Executor implementation notes"},{"code":"case T_SeqScan:\n\t\t\tpname = sname = \"Seq Scan\";\n\t\t\tbreak;","title":"EXPLAIN identity in core source"}],"tables":[{"columns":[{"key":"label","label":"Text-format label"},{"key":"identity","label":"Structured node identity"}],"key":"explain-labels","rows":[{"identity":"Seq Scan","label":"Seq Scan"}],"title":"EXPLAIN labels in this source build"}]}},"RequestedLocale":"en","Fallback":false,"Versions":["10","11","12","13","14","15","16","17","18","19","20"],"Locales":["en","zh-Hans"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}