{"Entry":{"collection":"plan","key":"subquery-scan","name":"Subquery Scan","aliases":["Subquery Scan","SubqueryScan","T_SubqueryScan"],"metadata":{"aliases":["Subquery Scan","SubqueryScan","T_SubqueryScan"],"category":"Scan","content_hash":"70363d6fcea0911fd594dda9d90bdf3225b7101dad569b9121dc58b13cc292fc","derived_content_hash":"f939c7e0896bf7d498cbf4e7e22ffb9f824f9a17fb4faa899d4e425d4f3a078e","imported_at":"2026-09-30T00:40:44.255582+08:00","name":"Subquery Scan","name_zh":"SubqueryScan","slug":"subquery-scan","summary":"Scans the output of a subquery plan."}},"Definition":{"Collection":"plan","Key":"subquery-scan","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"subquery-scan","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitSubqueryScan","memory_mechanism":"unclassified","node_tag":"T_SubqueryScan","parallel_callbacks":[],"partial_modes":[],"strategies":[],"text_names":["Subquery Scan"]},"comparison_hash":"2c67fdef6b55d5dfeedc05274c332f8e523a7b8dd3ffef1c7cb79b9d6d33566f","description":["Scans the output of a subquery plan."],"description_zh":"扫描子查询计划的输出。","description_zh_html":"\u003cp\u003e扫描子查询计划的输出。\u003c/p\u003e\u003cp\u003e核心节点标识：\u003ccode\u003eT_SubqueryScan\u003c/code\u003e；EXPLAIN 名称：\u003ccode\u003eSubquery 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：该节点实现中未提取到此类回调。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\u003cspan class=\"productname\"\u003ePostgreSQL\u003c/span\u003e 会为收到的每个查询制定一个\u003cem class=\"firstterm\"\u003e查询计划\u003c/em\u003e。选择与查询结构和数据特性相匹配的正确计划，对获得良好的性能至关重要，因此系统内置了一个复杂的\u003cem class=\"firstterm\"\u003e规划器\u003c/em\u003e来尽量选出好的计划。可以使用 \u003ca class=\"link\" href=\"/zh/docs/18/sql-explain.html\"\u003e\u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e\u003c/a\u003e 命令查看规划器为任意查询生成的查询计划。读懂计划是一门需要经验积累的技能，本节将尝试介绍其中的基础知识。\n   \u003c/p\u003e\u003cp\u003e\n    本节中的示例取自执行过 \u003ccode class=\"command\"\u003eVACUUM ANALYZE\u003c/code\u003e 的回归测试数据库，使用的是 v18 开发版本源码。如果自行尝试这些示例，通常应当能得到相近的结果，但估计代价和行计数可能略有差异，因为 \u003ccode class=\"command\"\u003eANALYZE\u003c/code\u003e 生成的统计信息来自随机采样而非精确计数，而且代价本身在某种程度上也依赖于平台。\n   \u003c/p\u003e\u003cp\u003e\n    这些示例使用 \u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e 默认的\u003cspan class=\"quote\"\u003e“\u003cspan class=\"quote\"\u003etext\u003c/span\u003e”\u003c/span\u003e输出格式，它紧凑且便于人工阅读。如果希望将 \u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e 的输出交给程序做进一步分析，则应改用机器可读的输出格式（XML、JSON 或 YAML）。\n   \u003c/p\u003e\u003cp\u003e\u003ca href=\"/zh/docs/18/using-explain.html\"\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":["Subquery Scan"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_SubqueryScan"},{"label":"Structured EXPLAIN Node Type","value":"Subquery Scan"},{"label":"Inputs","value":"A subquery child plan"},{"label":"Output","value":"Subquery result tuples"},{"label":"Executor initializer","value":"ExecInitSubqueryScan"},{"label":"Memory mechanism","value":"unclassified"}],"facts_zh":[{"label":"核心节点标签","value":"T_SubqueryScan"},{"label":"结构化 EXPLAIN 节点类型","value":"Subquery Scan"},{"label":"输入","value":"子查询的子计划"},{"label":"输出","value":"子查询结果元组"},{"label":"执行器初始化函数","value":"ExecInitSubqueryScan"},{"label":"内存机制","value":"unclassified"}],"memory":{"description":"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.","evidence":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeSubqueryscan.c","path":"src/backend/executor/nodeSubqueryscan.c","sha256":"9e5a1653e5b1678c457026f940a36bb2927cbfb1fa1f6a6a296e1bd1453ef29b","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"unclassified","source_notes":[]},"node_tag":"T_SubqueryScan","parallel_callbacks":[],"partial_modes":[],"related":[{"label":"EXPLAIN","url":"/wiki/sql/explain/?v=18"},{"label":"Using EXPLAIN","url":"/docs/18/using-explain.html"},{"label":"Parallel plans","url":"/docs/18/parallel-plans.html"}],"release":{"channel":"stable","label":"18.6","major":"18","ref":"PostgreSQL 18.6 source archive","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","source_snapshot_utc":"","source_url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},"runtime_verified":false,"sections":[{"paragraphs":["Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Subquery 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: none extracted from this node implementation."],"title":"Parallel execution and instrumentation"},{"paragraphs":["nodeSubqueryscan.c Support routines for scanning subqueries (subselects in rangetable).","This is just enough different from sublinks (nodeSubplan.c) to mean that we need two sets of code. Ought to look at trying to unify the cases.","We just return the subplan's result slot, rather than expending extra cycles for ExecCopySlot(). (Our own ScanTupleSlot is used only for EvalPlanQual rechecks.)","SubqueryRecheck -- access method routine to recheck a tuple in EvalPlanQual","Scans the subquery sequentially and returns the next qualifying tuple. We call the ExecScan() routine and pass it the appropriate access method functions."],"title":"Executor implementation notes"},{"code":"case T_SubqueryScan:\n\t\t\tpname = sname = \"Subquery 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：Subquery 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：none extracted from this node implementation."],"title":"并行执行与运行信息采集"},{"paragraphs":["nodeSubqueryscan.c：支持扫描子查询（范围表中的子 SELECT）的例程。","这与子链接（nodeSubplan.c）稍有差别，因此需要两套代码。应考虑尝试将这些情况统一。","直接返回子计划的结果槽位，避免 ExecCopySlot() 的额外开销。（节点自身的 ScanTupleSlot 仅用于 EvalPlanQual 重新检查。）","SubqueryRecheck：在 EvalPlanQual 中重新检查元组的访问方法例程。","顺序扫描子查询并返回下一个满足条件的元组。调用 ExecScan() 并传入相应的访问方法函数。"],"title":"执行器实现说明"},{"code":"case T_SubqueryScan:\n\t\t\tpname = sname = \"Subquery Scan\";\n\t\t\tbreak;","title":"核心源码中的 EXPLAIN 标识"}],"source_inventory":{"executor":"src/backend/executor/execProcnode.c","explain":"src/backend/commands/explain.c","implementation":"src/backend/executor/nodeSubqueryscan.c"},"sources":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/commands/explain.c:1462","line":1462,"path":"src/backend/commands/explain.c","sha256":"34c86d6070224a0e981efef51f79101d6d505e5874f1684ace183034bab14bb4","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/execProcnode.c:249","line":249,"path":"src/backend/executor/execProcnode.c","sha256":"f8a06a3f539077249b20664b2812433db6d7bd12b2c0ca633525db43d06f112a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeSubqueryscan.c","path":"src/backend/executor/nodeSubqueryscan.c","sha256":"9e5a1653e5b1678c457026f940a36bb2927cbfb1fa1f6a6a296e1bd1453ef29b","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/include/nodes/plannodes.h","path":"src/include/nodes/plannodes.h","sha256":"52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"strategies":[],"tables":[{"columns":[{"key":"label","label":"Text-format label"},{"key":"identity","label":"Structured node identity"}],"key":"explain-labels","rows":[{"identity":"Subquery Scan","label":"Subquery Scan"}],"title":"EXPLAIN labels in this source build"}],"tables_zh":[{"columns":[{"key":"label","label":"文本格式标签"},{"key":"identity","label":"结构化节点标识"}],"key":"explain-labels","rows":[{"identity":"Subquery Scan","label":"Subquery Scan"}],"title":"本构建中的 EXPLAIN 标签"}],"zh_from":{"authored_translation":"authored-translation-of-reviewed-guidance","body_sha256":"bbd6669c81029c18d1423d81f36ea2f0637abafd463480c2f676c9ec6fe6fd4e","kind":"same-version-chinese-manual-with-authored-translation-of-reviewed-guidance","manual":{"anchor":"","build_id":107,"extraction":"concept-introduction","fragment_sha256":"15fbdf84b8f499253ade4f4a5841de138619a9933b93cc241d431dbe922fa40c","locale":"zh-Hans","page_key":"using-explain.html","source_hash":"d2f749744b3ac5aa2eb0154f295e8163903cd20fcc130c74c6172cdf2ae3a9df","source_revision":"unknown:rendered-translation-baseline","translated_from_revision":null,"translation_revision":"git:b1b500221dbe5437cfca004d6b04d3f6e422765d;html-sha256:08e987a66af27b98444a8057c256314eafa13611a3fb9e888214fc436824244b","url":"/zh/docs/18/using-explain.html","version":"18"},"runtime_rerun":false,"source_facts_revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"}},"ManualEvidence":{"release":{"channel":"stable","label":"18.6","major":"18","ref":"PostgreSQL 18.6 source archive","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","source_snapshot_utc":"","source_url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},"sources":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/commands/explain.c:1462","line":1462,"path":"src/backend/commands/explain.c","sha256":"34c86d6070224a0e981efef51f79101d6d505e5874f1684ace183034bab14bb4","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/execProcnode.c:249","line":249,"path":"src/backend/executor/execProcnode.c","sha256":"f8a06a3f539077249b20664b2812433db6d7bd12b2c0ca633525db43d06f112a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeSubqueryscan.c","path":"src/backend/executor/nodeSubqueryscan.c","sha256":"9e5a1653e5b1678c457026f940a36bb2927cbfb1fa1f6a6a296e1bd1453ef29b","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/include/nodes/plannodes.h","path":"src/include/nodes/plannodes.h","sha256":"52422b327a8049fbbb20d8b96008a0fc0a6fafa60f7eff3c695d5b2e83830120","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}]},"MeasuredEvidence":{"runtime_verified":false}},"Text":{"Collection":"plan","Key":"subquery-scan","SourceDatabase":"center","Version":"18","Locale":"en","Title":"Subquery Scan","Summary":"Scans the output of a subquery plan.","BodyHTML":"\u003cp\u003eScans the output of a subquery plan.\u003c/p\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"3232dab55d95f3f1cabb8ca78ee394a902cd672718394408c71c45165d700aac","Payload":{"description":["Scans the output of a subquery plan."],"related":[{"label":"EXPLAIN","url":"/wiki/sql/explain/?v=18"},{"label":"Using EXPLAIN","url":"/docs/18/using-explain.html"},{"label":"Parallel plans","url":"/docs/18/parallel-plans.html"}],"sections":[{"paragraphs":["Structured formats use the Node Type above. Text-format spellings can also include operation, strategy, join type, scan direction or aggregation-stage attributes.","Text names recorded by this source: Subquery 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: none extracted from this node implementation."],"title":"Parallel execution and instrumentation"},{"paragraphs":["nodeSubqueryscan.c Support routines for scanning subqueries (subselects in rangetable).","This is just enough different from sublinks (nodeSubplan.c) to mean that we need two sets of code. Ought to look at trying to unify the cases.","We just return the subplan's result slot, rather than expending extra cycles for ExecCopySlot(). (Our own ScanTupleSlot is used only for EvalPlanQual rechecks.)","SubqueryRecheck -- access method routine to recheck a tuple in EvalPlanQual","Scans the subquery sequentially and returns the next qualifying tuple. We call the ExecScan() routine and pass it the appropriate access method functions."],"title":"Executor implementation notes"},{"code":"case T_SubqueryScan:\n\t\t\tpname = sname = \"Subquery 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":"Subquery Scan","label":"Subquery 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}