{"Entry":{"collection":"plan","key":"materialize","name":"Materialize","aliases":["Material","Materialize","T_Material"],"metadata":{"aliases":["Material","Materialize","T_Material"],"category":"Materialization","content_hash":"69abc3bfb2a3c9678e45280109d1dc0cab0cacb9b0c08d2ee0cd6c581d2154be","derived_content_hash":"739ca55c2e81f5b40986bb6c4a7115ad605183e832a7a188783c15457ee0ddcd","imported_at":"2026-09-30T00:40:44.151445+08:00","name":"Materialize","name_zh":"Material","slug":"materialize","summary":"Stores its child output so the rows can be read again."}},"Definition":{"Collection":"plan","Key":"materialize","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"materialize","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitMaterial","memory_mechanism":"tuplestore","node_tag":"T_Material","parallel_callbacks":[],"partial_modes":[],"strategies":[],"text_names":["Materialize"]},"comparison_hash":"c1d37bf334409fe6ded8ce560fe45fd495ff89ceda2808b6f5ef18daca520393","description":["Stores its child output so the rows can be read again."],"description_zh":"保存子计划输出，使这些行可以再次读取。","description_zh_html":"\u003cp\u003e保存子计划输出，使这些行可以再次读取。\u003c/p\u003e\u003cp\u003e核心节点标识：\u003ccode\u003eT_Material\u003c/code\u003e；EXPLAIN 名称：\u003ccode\u003eMaterialize\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\u003cp\u003e该节点使用受 \u003ccode\u003ework_mem\u003c/code\u003e 影响的元组存储，必要时可将存储的元组写入临时文件。这并不表示 \u003ccode\u003ework_mem\u003c/code\u003e 限制了该节点的所有内存分配。\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":["Materialize"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_Material"},{"label":"Structured EXPLAIN Node Type","value":"Materialize"},{"label":"Inputs","value":"One child plan"},{"label":"Output","value":"Materialized tuples"},{"label":"Executor initializer","value":"ExecInitMaterial"},{"label":"Memory mechanism","value":"tuplestore"}],"facts_zh":[{"label":"核心节点标签","value":"T_Material"},{"label":"结构化 EXPLAIN 节点类型","value":"Materialize"},{"label":"输入","value":"一个子计划"},{"label":"输出","value":"已物化的元组"},{"label":"执行器初始化函数","value":"ExecInitMaterial"},{"label":"内存机制","value":"tuplestore"}],"memory":{"description":"This node creates a tuplestore with work_mem. The tuplestore can move stored tuples to temporary files; this does not make work_mem a cap on every allocation made by the node.","evidence":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeMaterial.c","path":"src/backend/executor/nodeMaterial.c","sha256":"410b7129ca7e86ce3cf59ed7afed813fc6a47d68f5195e14df36559c75c7ef91","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/utils/sort/tuplestore.c","path":"src/backend/utils/sort/tuplestore.c","sha256":"27a85416bafed557006c705ab5d0c5dce3d3c1702812bb54d136f2ac4c41055a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"tuplestore","source_notes":[]},"node_tag":"T_Material","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"},{"label":"enable_material","url":"/wiki/guc/enable_material/?v=18"},{"label":"work_mem","url":"/wiki/guc/work_mem/?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: Materialize.","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 node creates a tuplestore with work_mem. The tuplestore can move stored tuples to temporary files; this does not make work_mem a cap on every allocation made by the node."],"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":["Notice that here the planner has chosen to “ materialize ” the inner relation of the join, by putting a Materialize plan node atop it. This means that the t2 index scan will be done just once, even though the nested-loop join node needs to read that data ten times, once for each row from the outer relation. The Materialize node saves the data in memory as it's read, and then returns the data from memory on each subsequent pass."],"title":"Same-version manual discussion"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t1, tenk2 t2\nWHERE t1.unique1 \u003c 10 AND t2.unique2 \u003c 10 AND t1.hundred \u003c t2.hundred;\n\n                                         QUERY PLAN\n---------------------------------------------------------------------------------------------\n Nested Loop  (cost=4.65..49.36 rows=33 width=488)\n   Join Filter: (t1.hundred \u003c t2.hundred)\n   -\u003e  Bitmap Heap Scan on tenk1 t1  (cost=4.36..39.38 rows=10 width=244)\n         Recheck Cond: (unique1 \u003c 10)\n         -\u003e  Bitmap Index Scan on tenk1_unique1  (cost=0.00..4.36 rows=10 width=0)\n               Index Cond: (unique1 \u003c 10)\n   -\u003e  Materialize  (cost=0.29..8.51 rows=10 width=244)\n         -\u003e  Index Scan using tenk2_unique2 on tenk2 t2  (cost=0.29..8.46 rows=10 width=244)\n               Index Cond: (unique2 \u003c 10)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","In this example the join's output row count is the same as the product of the two scans' row counts, but that's not true in all cases because there can be additional WHERE clauses that mention both tables and so can only be applied at the join point, not to either input scan. Here's an example:"],"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":["As long as we are at the end of the data collected in the tuplestore, we collect one new row from the subplan on each call, and stash it aside in the tuplestore before returning it. The tuplestore is only read if we are asked to scan backwards, rescan, or mark/restore.","If first time through, and we need a tuplestore, initialize it.","Allocate a second read pointer to serve as the mark. We know it must have index 1, so needn't store that.","If we are not at the end of the tuplestore, or are going backwards, try to fetch a tuple from tuplestore.","When reversing direction at tuplestore EOF, the first gettupleslot call will fetch the last-added tuple; but we want to return the one before that, if possible. So do an extra fetch."],"title":"Executor implementation notes"},{"code":"case T_Material:\n\t\t\tpname = sname = \"Materialize\";\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：Materialize.","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":["此节点使用 work_mem 创建 tuplestore。tuplestore 可以将存储的元组移到临时文件，但这不意味着 work_mem 限制了节点的全部内存分配。"],"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":["规划器在连接内侧关系之上加入 Materialize 节点，将其物化。因此，尽管嵌套循环连接需要为外侧的每一行读取一次内侧数据、共十次，t2 的索引扫描也只执行一次。Materialize 在读取时将数据保存在内存中，后续各轮直接从内存返回数据。"],"title":"同版本手册说明"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t1, tenk2 t2\nWHERE t1.unique1 \u003c 10 AND t2.unique2 \u003c 10 AND t1.hundred \u003c t2.hundred;\n\n                                         QUERY PLAN\n---------------------------------------------------------------------------------------------\n Nested Loop  (cost=4.65..49.36 rows=33 width=488)\n   Join Filter: (t1.hundred \u003c t2.hundred)\n   -\u003e  Bitmap Heap Scan on tenk1 t1  (cost=4.36..39.38 rows=10 width=244)\n         Recheck Cond: (unique1 \u003c 10)\n         -\u003e  Bitmap Index Scan on tenk1_unique1  (cost=0.00..4.36 rows=10 width=0)\n               Index Cond: (unique1 \u003c 10)\n   -\u003e  Materialize  (cost=0.29..8.51 rows=10 width=244)\n         -\u003e  Index Scan using tenk2_unique2 on tenk2 t2  (cost=0.29..8.46 rows=10 width=244)\n               Index Cond: (unique2 \u003c 10)","paragraphs":["示例摘自 PostgreSQL 18.6 手册；本百科未实际执行此示例。","本例中，连接的输出行数等于两次扫描行数的乘积，但并非总是如此：可能存在同时引用两个表的额外 WHERE 子句，只能在连接时应用，无法应用到任何单侧输入扫描。示例如下："],"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":["只要当前位于 tuplestore 已收集数据的末尾，每次调用就从子计划获取一行新数据，在返回之前将其存入 tuplestore。只有请求反向扫描、重扫或标记／恢复时，才从 tuplestore 读取。","首次执行且需要元组存储时，初始化它。","分配第二个读取指针作为标记。已知其索引必为 1，因此无需另行保存。","如果尚未到达 tuplestore 末尾，或正在反向扫描，则尝试从 tuplestore 获取元组。","在元组存储末尾反转扫描方向时，第一次 gettupleslot 会取得最后添加的元组；但若可能，我们希望返回它之前的那个，因此额外读取一次。"],"title":"执行器实现说明"},{"code":"case T_Material:\n\t\t\tpname = sname = \"Materialize\";\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/nodeMaterial.c"},"sources":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/commands/explain.c:1516","line":1516,"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:315","line":315,"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/nodeMaterial.c","path":"src/backend/executor/nodeMaterial.c","sha256":"410b7129ca7e86ce3cf59ed7afed813fc6a47d68f5195e14df36559c75c7ef91","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"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/utils/sort/tuplestore.c","path":"src/backend/utils/sort/tuplestore.c","sha256":"27a85416bafed557006c705ab5d0c5dce3d3c1702812bb54d136f2ac4c41055a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}],"strategies":[],"tables":[{"columns":[{"key":"label","label":"Text-format label"},{"key":"identity","label":"Structured node identity"}],"key":"explain-labels","rows":[{"identity":"Materialize","label":"Materialize"}],"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":"Materialize","label":"Materialize"}],"title":"本构建中的 EXPLAIN 标签"}],"zh_from":{"authored_translation":"authored-translation-of-reviewed-guidance","body_sha256":"7efe0f151e725a0778eabbc6c515ccf96eac67156b5a2341b634693140032e47","kind":"same-version-chinese-manual-with-authored-translation-of-reviewed-guidance","manual":{"anchor":"USING-EXPLAIN-BASICS","build_id":107,"extraction":"concept-introduction","fragment_sha256":"bc79b6f9c70f2b56968e08ea26d2c9ec3d395e425aca0594e227203daa0ad29b","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#USING-EXPLAIN-BASICS","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:1516","line":1516,"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:315","line":315,"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/nodeMaterial.c","path":"src/backend/executor/nodeMaterial.c","sha256":"410b7129ca7e86ce3cf59ed7afed813fc6a47d68f5195e14df36559c75c7ef91","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"},{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/utils/sort/tuplestore.c","path":"src/backend/utils/sort/tuplestore.c","sha256":"27a85416bafed557006c705ab5d0c5dce3d3c1702812bb54d136f2ac4c41055a","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"},{"label":"PostgreSQL 18.6 · using-explain","path":"using-explain.html","sha256":"60040c30180093418a0affe56dd27dff9df2504b705b38039589e458bf5c31ed","url":"/docs/18/using-explain.html#USING-EXPLAIN-BASICS"}]},"MeasuredEvidence":{"runtime_verified":false}},"Text":{"Collection":"plan","Key":"materialize","SourceDatabase":"center","Version":"18","Locale":"en","Title":"Materialize","Summary":"Stores its child output so the rows can be read again.","BodyHTML":"\u003cp\u003eStores its child output so the rows can be read again.\u003c/p\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"8a8b6b19e46f2b8aae4cd4715eb4acba1ae56cc093392e516243faa97df9b01d","Payload":{"description":["Stores its child output so the rows can be read again."],"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_material","url":"/wiki/guc/enable_material/?v=18"},{"label":"work_mem","url":"/wiki/guc/work_mem/?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: Materialize.","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 node creates a tuplestore with work_mem. The tuplestore can move stored tuples to temporary files; this does not make work_mem a cap on every allocation made by the node."],"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":["Notice that here the planner has chosen to “ materialize ” the inner relation of the join, by putting a Materialize plan node atop it. This means that the t2 index scan will be done just once, even though the nested-loop join node needs to read that data ten times, once for each row from the outer relation. The Materialize node saves the data in memory as it's read, and then returns the data from memory on each subsequent pass."],"title":"Same-version manual discussion"},{"blocks":[{"code":"EXPLAIN SELECT *\nFROM tenk1 t1, tenk2 t2\nWHERE t1.unique1 \u003c 10 AND t2.unique2 \u003c 10 AND t1.hundred \u003c t2.hundred;\n\n                                         QUERY PLAN\n---------------------------------------------------------------------------------------------\n Nested Loop  (cost=4.65..49.36 rows=33 width=488)\n   Join Filter: (t1.hundred \u003c t2.hundred)\n   -\u003e  Bitmap Heap Scan on tenk1 t1  (cost=4.36..39.38 rows=10 width=244)\n         Recheck Cond: (unique1 \u003c 10)\n         -\u003e  Bitmap Index Scan on tenk1_unique1  (cost=0.00..4.36 rows=10 width=0)\n               Index Cond: (unique1 \u003c 10)\n   -\u003e  Materialize  (cost=0.29..8.51 rows=10 width=244)\n         -\u003e  Index Scan using tenk2_unique2 on tenk2 t2  (cost=0.29..8.46 rows=10 width=244)\n               Index Cond: (unique2 \u003c 10)","paragraphs":["Example copied from the PostgreSQL 18.6 manual; it was not executed for this collection.","In this example the join's output row count is the same as the product of the two scans' row counts, but that's not true in all cases because there can be additional WHERE clauses that mention both tables and so can only be applied at the join point, not to either input scan. Here's an example:"],"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":["As long as we are at the end of the data collected in the tuplestore, we collect one new row from the subplan on each call, and stash it aside in the tuplestore before returning it. The tuplestore is only read if we are asked to scan backwards, rescan, or mark/restore.","If first time through, and we need a tuplestore, initialize it.","Allocate a second read pointer to serve as the mark. We know it must have index 1, so needn't store that.","If we are not at the end of the tuplestore, or are going backwards, try to fetch a tuple from tuplestore.","When reversing direction at tuplestore EOF, the first gettupleslot call will fetch the last-added tuple; but we want to return the one before that, if possible. So do an extra fetch."],"title":"Executor implementation notes"},{"code":"case T_Material:\n\t\t\tpname = sname = \"Materialize\";\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":"Materialize","label":"Materialize"}],"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}