{"Entry":{"collection":"plan","key":"index-only-scan","name":"Index Only Scan","aliases":["Index Only Scan","IndexOnlyScan","T_IndexOnlyScan"],"metadata":{"aliases":["Index Only Scan","IndexOnlyScan","T_IndexOnlyScan"],"category":"Scan","content_hash":"788b6c7dbf8a0476e6e73fe8a750a4dccf52593f2222ae4a01b19bf08c7385d4","derived_content_hash":"d1b857c74d8dbc547d79e7e02845e7dbe2d6af15a0bbae99c10428e378a1fded","imported_at":"2026-09-30T00:40:44.222116+08:00","name":"Index Only Scan","name_zh":"IndexOnlyScan","slug":"index-only-scan","summary":"Uses index values for the result and checks tuple visibility, visiting the heap when the visibility map does not suffice."}},"Definition":{"Collection":"plan","Key":"index-only-scan","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"index-only-scan","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitIndexOnlyScan","memory_mechanism":"unclassified","node_tag":"T_IndexOnlyScan","parallel_callbacks":["ExecIndexOnlyScanEstimate","ExecIndexOnlyScanInitializeDSM","ExecIndexOnlyScanInitializeWorker","ExecIndexOnlyScanReInitializeDSM","ExecIndexOnlyScanRetrieveInstrumentation"],"partial_modes":[],"strategies":[],"text_names":["Index Only Scan"]},"comparison_hash":"149a13d2842f396b0e35eb23a93337d9e8c708fbcf6665eec0ab0ebbd5320b9b","description":["Uses index values for the result and checks tuple visibility, visiting the heap when the visibility map does not suffice."],"description_zh":"利用索引值生成结果并检查可见性；可见性映射不足以判断时仍访问堆。","description_zh_html":"\u003cp\u003e利用索引值生成结果并检查可见性；可见性映射不足以判断时仍访问堆。\u003c/p\u003e\u003cp\u003e核心节点标识：\u003ccode\u003eT_IndexOnlyScan\u003c/code\u003e；EXPLAIN 名称：\u003ccode\u003eIndex Only 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\u003eExecIndexOnlyScanEstimate, ExecIndexOnlyScanInitializeDSM, ExecIndexOnlyScanInitializeWorker, ExecIndexOnlyScanReInitializeDSM, ExecIndexOnlyScanRetrieveInstrumentation\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":["Index Only Scan"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_IndexOnlyScan"},{"label":"Structured EXPLAIN Node Type","value":"Index Only Scan"},{"label":"Inputs","value":"An index, relation and visibility information"},{"label":"Output","value":"Qualified tuples from index values"},{"label":"Executor initializer","value":"ExecInitIndexOnlyScan"},{"label":"Memory mechanism","value":"unclassified"}],"facts_zh":[{"label":"核心节点标签","value":"T_IndexOnlyScan"},{"label":"结构化 EXPLAIN 节点类型","value":"Index Only Scan"},{"label":"输入","value":"索引、关系及可见性信息"},{"label":"输出","value":"来自索引值且满足条件的元组"},{"label":"执行器初始化函数","value":"ExecInitIndexOnlyScan"},{"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/nodeIndexonlyscan.c","path":"src/backend/executor/nodeIndexonlyscan.c","sha256":"357d4ddbd6ba5d9b02f04185e579e2f2dc889d0d94875580770c8649cc427e6d","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"unclassified","source_notes":[]},"node_tag":"T_IndexOnlyScan","parallel_callbacks":["ExecIndexOnlyScanEstimate","ExecIndexOnlyScanInitializeDSM","ExecIndexOnlyScanInitializeWorker","ExecIndexOnlyScanReInitializeDSM","ExecIndexOnlyScanRetrieveInstrumentation"],"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"},{"label":"enable_indexonlyscan","url":"/wiki/guc/enable_indexonlyscan/?v=18"},{"label":"random_page_cost","url":"/wiki/guc/random_page_cost/?v=18"},{"label":"Index AM","url":"/wiki/indexam/?v=18"}],"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: Index Only 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: ExecIndexOnlyScanEstimate, ExecIndexOnlyScanInitializeDSM, ExecIndexOnlyScanInitializeWorker, ExecIndexOnlyScanReInitializeDSM, ExecIndexOnlyScanRetrieveInstrumentation."],"title":"Parallel execution and instrumentation"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t\nWHERE t.unique1 NOT IN (SELECT o.unique1 FROM onek o);\n\n                                         QUERY PLAN\n--------------------------------------------------------------------------------------------\n Seq Scan on tenk1 t  (cost=61.77..531.77 rows=5000 width=244)\n   Filter: (NOT (ANY (unique1 = (hashed SubPlan 1).col1)))\n   SubPlan 1\n     -\u003e  Index Only Scan using onek_unique1 on onek o  (cost=0.28..59.27 rows=1000 width=4)\n(4 rows)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","In the example above, the ALL operator runs the subplan again for each row of the outer query (which accounts for the high estimated cost). Some queries can use a hashed subplan to avoid that:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN ANALYZE SELECT four, unique1 FROM tenk1 WHERE four BETWEEN 1 AND 3 AND unique1 = 42;\n                                                              QUERY PLAN\n----------------------------------------------------------------------------------------------------------------------------------\n Index Only Scan using tenk1_four_unique1_idx on tenk1  (cost=0.29..6.90 rows=1 width=8) (actual time=0.006..0.007 rows=1.00 loops=1)\n   Index Cond: ((four \u003e= 1) AND (four \u003c= 3) AND (unique1 = 42))\n   Heap Fetches: 0\n   Index Searches: 3\n   Buffers: shared hit=7\n Planning Time: 0.029 ms\n Execution Time: 0.012 ms","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","The “ Index Searches ” line is also useful with B-tree index scans that apply the skip scan optimization to more efficiently traverse through an index:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-ANALYZE"}}],"title":"Examples from this manual build"},{"paragraphs":["Determine which direction to scan the index in based on the plan's scan direction and the current direction of execution.","We reach here if the index only scan is not parallel, or if we're serially executing an index only scan that was planned to be parallel.","If no run-time keys to calculate or they are ready, go ahead and pass the scankeys to the index AM.","We can skip the heap fetch if the TID references a heap page on which all tuples are known visible to everybody. In any case, we'll use the index tuple not the heap tuple as the data source.","Note on Memory Ordering Effects: visibilitymap_get_status does not lock the visibility map buffer, and therefore the result we read here could be slightly stale. However, it can't be stale enough to matter."],"title":"Executor implementation notes"},{"code":"case T_IndexOnlyScan:\n\t\t\tpname = sname = \"Index Only 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：Index Only 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：ExecIndexOnlyScanEstimate, ExecIndexOnlyScanInitializeDSM, ExecIndexOnlyScanInitializeWorker, ExecIndexOnlyScanReInitializeDSM, ExecIndexOnlyScanRetrieveInstrumentation."],"title":"并行执行与运行信息采集"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t\nWHERE t.unique1 NOT IN (SELECT o.unique1 FROM onek o);\n\n                                         QUERY PLAN\n--------------------------------------------------------------------------------------------\n Seq Scan on tenk1 t  (cost=61.77..531.77 rows=5000 width=244)\n   Filter: (NOT (ANY (unique1 = (hashed SubPlan 1).col1)))\n   SubPlan 1\n     -\u003e  Index Only Scan using onek_unique1 on onek o  (cost=0.28..59.27 rows=1000 width=4)\n(4 rows)","paragraphs":["示例摘自 PostgreSQL 18.6 手册；本百科未实际执行此示例。","在上面的例子中， ALL 操作符会对外层查询的每一行都重新执行一次子计划（这也是估计代价很高的原因）。有些查询可以使用 hash 子计划 来避免这一点："],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN ANALYZE SELECT four, unique1 FROM tenk1 WHERE four BETWEEN 1 AND 3 AND unique1 = 42;\n                                                              QUERY PLAN\n----------------------------------------------------------------------------------------------------------------------------------\n Index Only Scan using tenk1_four_unique1_idx on tenk1  (cost=0.29..6.90 rows=1 width=8) (actual time=0.006..0.007 rows=1.00 loops=1)\n   Index Cond: ((four \u003e= 1) AND (four \u003c= 3) AND (unique1 = 42))\n   Heap Fetches: 0\n   Index Searches: 3\n   Buffers: shared hit=7\n Planning Time: 0.029 ms\n Execution Time: 0.012 ms","paragraphs":["示例摘自 PostgreSQL 18.6 手册；本百科未实际执行此示例。","“ Index Searches ” 这一行对于应用了 跳过扫描 （skip scan）优化、以更高效方式遍历索引的 B-树索引扫描同样有用："],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-ANALYZE"}}],"title":"本版手册中的示例"},{"paragraphs":["根据计划的扫描方向和当前执行方向，确定索引扫描方向。","仅索引扫描不并行执行，或原计划并行的仅索引扫描以串行方式执行时，会到达此处。","如果没有需要计算的运行时键，或运行时键已准备就绪，则将扫描键传给索引访问方法。","若 TID 指向的堆页面上所有元组都已知对所有事务可见，就可以跳过堆读取。无论是否读取堆，都使用索引元组而非堆元组作为数据来源。","关于内存序：visibilitymap_get_status 不锁定可见性映射缓冲区，因此此处读到的结果可能稍有滞后，但不会滞后到影响正确性。"],"title":"执行器实现说明"},{"code":"case T_IndexOnlyScan:\n\t\t\tpname = sname = \"Index Only Scan\";\n\t\t\tbreak;","title":"核心源码中的 EXPLAIN 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suffice.\u003c/p\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"213c9bc35ee0182fc346fcecc61380d394e6091b775a06f3897321932a2f20c9","Payload":{"description":["Uses index values for the result and checks tuple visibility, visiting the heap when the visibility map does not suffice."],"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"},{"label":"enable_indexonlyscan","url":"/wiki/guc/enable_indexonlyscan/?v=18"},{"label":"random_page_cost","url":"/wiki/guc/random_page_cost/?v=18"},{"label":"Index AM","url":"/wiki/indexam/?v=18"}],"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: Index Only 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: ExecIndexOnlyScanEstimate, ExecIndexOnlyScanInitializeDSM, ExecIndexOnlyScanInitializeWorker, ExecIndexOnlyScanReInitializeDSM, ExecIndexOnlyScanRetrieveInstrumentation."],"title":"Parallel execution and instrumentation"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t\nWHERE t.unique1 NOT IN (SELECT o.unique1 FROM onek o);\n\n                                         QUERY PLAN\n--------------------------------------------------------------------------------------------\n Seq Scan on tenk1 t  (cost=61.77..531.77 rows=5000 width=244)\n   Filter: (NOT (ANY (unique1 = (hashed SubPlan 1).col1)))\n   SubPlan 1\n     -\u003e  Index Only Scan using onek_unique1 on onek o  (cost=0.28..59.27 rows=1000 width=4)\n(4 rows)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","In the example above, the ALL operator runs the subplan again for each row of the outer query (which accounts for the high estimated cost). Some queries can use a hashed subplan to avoid that:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}},{"code":"EXPLAIN ANALYZE SELECT four, unique1 FROM tenk1 WHERE four BETWEEN 1 AND 3 AND unique1 = 42;\n                                                              QUERY PLAN\n----------------------------------------------------------------------------------------------------------------------------------\n Index Only Scan using tenk1_four_unique1_idx on tenk1  (cost=0.29..6.90 rows=1 width=8) (actual time=0.006..0.007 rows=1.00 loops=1)\n   Index Cond: ((four \u003e= 1) AND (four \u003c= 3) AND (unique1 = 42))\n   Heap Fetches: 0\n   Index Searches: 3\n   Buffers: shared hit=7\n Planning Time: 0.029 ms\n Execution Time: 0.012 ms","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","The “ Index Searches ” line is also useful with B-tree index scans that apply the skip scan optimization to more efficiently traverse through an index:"],"source":{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-ANALYZE"}}],"title":"Examples from this manual build"},{"paragraphs":["Determine which direction to scan the index in based on the plan's scan direction and the current direction of execution.","We reach here if the index only scan is not parallel, or if we're serially executing an index only scan that was planned to be parallel.","If no run-time keys to calculate or they are ready, go ahead and pass the scankeys to the index AM.","We can skip the heap fetch if the TID references a heap page on which all tuples are known visible to everybody. In any case, we'll use the index tuple not the heap tuple as the data source.","Note on Memory Ordering Effects: visibilitymap_get_status does not lock the visibility map buffer, and therefore the result we read here could be slightly stale. However, it can't be stale enough to matter."],"title":"Executor implementation notes"},{"code":"case T_IndexOnlyScan:\n\t\t\tpname = sname = \"Index Only 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":"Index Only Scan","label":"Index Only 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}