From 589a6ab1f0700364f1caf37c046aaf5b8914ab09 Mon Sep 17 00:00:00 2001
From: jason-price-mongodb <jshfjghsdfgjsdjh@aolsdjfhkjsdhfkjsdf.com>
Date: Tue, 17 Aug 2021 17:31:24 -0700
Subject: [PATCH] DOCS-14453 execution stats inner lookup

DOCS-14453 execution stats inner lookup
---
 source/indexes.txt                   |  2 +
 source/reference/explain-results.txt | 69 ++++++++++++++++++++++++++++
 source/release-notes/5.0.txt         |  7 +++
 3 files changed, 78 insertions(+)

diff --git a/source/indexes.txt b/source/indexes.txt
index 2d3458a1b5e..eb6a7646d2f 100644
--- a/source/indexes.txt
+++ b/source/indexes.txt
@@ -295,6 +295,8 @@ page </reference/collation/>`.
 
 .. include:: /includes/extracts/collation-index-type-restrictions.rst
 
+.. _indexes-covered-queries:
+
 Covered Queries
 ---------------
 
diff --git a/source/reference/explain-results.txt b/source/reference/explain-results.txt
index 72ff9c76833..510fa93a6fc 100644
--- a/source/reference/explain-results.txt
+++ b/source/reference/explain-results.txt
@@ -1,3 +1,5 @@
+.. _explain-results:
+
 ===============
 Explain Results
 ===============
@@ -605,6 +607,73 @@ execution of the winning plan. In order to include
       both the winning and rejected plans. The field is present only if
       ``explain`` runs in ``allPlansExecution`` verbosity mode.
 
+.. _explain-results-lookup:
+
+Execution Plan Statistics for Query with ``$lookup`` Pipeline Stage
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 5.0
+
+The :ref:`explain results <explain-results>` can include execution
+statistics for queries that use a :pipeline:`$lookup` pipeline stage. To
+include those execution statistics, you must run the explain operation
+in one of these execution verbosity modes:
+
+- :ref:`executionStats <explain-method-executionStats>`
+- :ref:`allPlansExecution <explain-method-allPlansExecution>`
+
+The following fields are included in the explain results for a
+:pipeline:`$lookup` query:
+
+.. code-block:: none
+   :copyable: false
+
+   '$lookup': {
+      from: <string>,
+      as: <string>,
+      localField: <string>,
+      foreignField: <string>
+   },
+   totalDocsExamined: <long>,
+   totalKeysExamined: <long>,
+   collectionScans: <long>,
+   indexesUsed: [ <string_1>, <string_2>, ..., <string_n> ],
+   nReturned: <long>,
+   executionTimeMillisEstimate: <long>
+
+To see the descriptions for the fields in the ``$lookup`` section, see
+the :pipeline:`$lookup` page.
+
+The other fields are:
+
+.. data:: explain.totalDocsExamined
+
+   Number of documents examined during the query execution.
+
+.. data:: explain.totalKeysExamined
+
+   Number of index keys examined.
+
+.. data:: explain.collectionScans
+
+   Number of times a collection scan occurred during query execution.
+   During a collection scan, each document in a collection is compared
+   to the query predicate. Collection scans occur if no appropriate
+   :ref:`index <indexes>` exists that :ref:`covers
+   <indexes-covered-queries>` the query.
+
+.. data:: explain.indexesUsed
+
+   Array of strings with the names of the indexes used by the query.
+
+.. data:: explain.nReturned
+
+   Number of documents that match the query condition.
+
+.. data:: explain.executionTimeMillisEstimate
+
+   Estimated time in milliseconds for the query execution.
+
 .. _serverInfo:
 
 ``serverInfo``
diff --git a/source/release-notes/5.0.txt b/source/release-notes/5.0.txt
index 9c79cb4a55b..1c524aa2a88 100644
--- a/source/release-notes/5.0.txt
+++ b/source/release-notes/5.0.txt
@@ -597,6 +597,13 @@ Transactions
 General Improvements
 --------------------
 
+Execution Plan Statistics for Query with ``$lookup`` Pipeline Stage
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+MongoDB 5.0 adds :ref:`execution plan statistics
+<explain-results-lookup>` for queries that use a :pipeline:`$lookup`
+pipeline stage.
+
 Improved Handling of (``$``) and (``.``) in Field Names
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~