Explain Plan Steps

ExplainPlanSteps.md

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+**MongoDB Explain() plan steps**
+
+Interpreting explain plans is a core competency for any MongoDB
+performance practitioner, and we've seen many examples of explain plans
+in the book. In this appendix, we list all of the individual step
+definitions that we are aware of and give a brief explanation of each.
+
+|Step|Description|
+|--- | -------------------------------------------------- |
+|**AND_HASH**                 | Merge two or more index outputs for an index intersection plan. 
+|**AND_SORTED**                |Merge two or more index outputs for an index intersection plan.
+|**CACHED_PLAN**|               Indicates that a plan was retrieved from the plan cache rather than from a real-time optimizer action.
+|  **COLLSCAN**  |                Read every document in the collection.
+|  **COUNT**      |               Count the documents supplied by the inut step.
+|  **COUNT_SCAN**  |              A quick count of the documents returned by an index scan.
+|  **DELETE**       |             Documents are being deleted
+|  **DISTINCT_SCAN** |            An index scan that returns only unique values for a key
+|  **ENSURE_SORTED**  |           Checks the output of the previous stage to ensure that it is in the expected sorted order
+|  **EOF** |                      Usually means that the collection queried does not exist.
+|  **FETCH**|                     Get documents from a collection. This usually occurs after in index scan when additional attributes are required for filtering or projecting.
+|  **GEO_NEAR_2D**|               Get documents using a geospatial query against a 2d index to calculate geometries. Intended for legacy coordinate pairs in earlier versions of MongoDB.
+|  **GEO_NEAR_2DSPHERE** |        Get documents using a geospatial query against a 2dsphere index to calculate geometries on a sphere. More commonly found in modern versions of MongoDB.
+ | **IDHACK** |                   Get a document using the "\_id" attribute.
+|  **IXSCAN**  |                  An index is scanned looking for matching documents or to return documents in sorted order.
+ | **LIMIT**    |                 Restricts the number of documents returned in the subsequent stages.
+ | **MOCK**      |                Used only in unit tests to mock results for testing.
+ | **MULTI_ITERATOR** |           Iterates over a collection. This stage is commonly seen in \$sample stages of aggregations.
+ | **MULTI_PLAN** |               Multiple query plans were evaluated during command execution
+ | **OR**         |               Two results were merged for a \$or operations -- usually associated with an index merge.
+ | **PROJECTION_COVERED** |        An explicit projection which is supported by an index scan (eg, for a "covered" index query).
+ | **PROJECTION_DEFAULT**  |      A "default" projection, where no explicit projection is requested.
+ | **PROJECTION_SIMPLE**   |      An explicit projection which is supported by a collection access -- usually preceded by a FETCH or COLLSCAN step.
+|  **QUEUED_DATA**         |      Usually associated with a GetMore operation that retrieves data from an open cursor.
+|  **RECORD_STORE_FAST_COUNT** |  Access the MongoDB "fast count" value for a collection -- to avoid having to scan the collection when a document count is requested.
+ | **RETURN_KEY** |               Indicates that the returnkey() modifier was issued, to return index key values only.
+  |**SHARD_MERGE** |              Data from multiple shards was merged.
+ | **SHARDING_FILTER** |          Data is returned from an individual shard to the mongos process.
+ | **SKIP**  |                    Documents were skipped.
+ | **SORT**   |                   Documents from the previous step where sorted.
+ | **SORT_KEY_GENERATOR** |       Keys from the previous step are extracted to be fed into a subsequent sort step.
+ | **SORT_MERGE** |               An index merge in which the results from multiple index scans are sorted and merged
+ | **SUBPLAN**    |               The outputs of multiple query plans (usually index scans) are combined to support an \$or operation.
+ | **TEXT**        |              Returns documents that have resulted from a text search.
+ | **TEXT_MATCH**   |             Returns any documents output from the previous stage matching a given text search query.
+ | **TEXT_OR**       |            Returns documents containing positive terms in a text search query, along with their scores.
+ | **TRIAL**          |           Stage for choosing between two alternate plans based on an initial trial period.
+ | **UPDATE**          |          Documents are updated.
+