# Reading indexed `$or` plans with explain > Source-anchored research for Reading indexed `$or` plans with explain. Run status: COMPLETED. Parent: [Compound index vs index union for $or](https://llms-explorer.com/tree/compound-index-vs-index-union-for-or/) · 6 facets · 18 facts · page: https://llms-explorer.com/tree/reading-indexed-or-plans-with-explain/ ## Definitions - An indexed query is not necessarily covered. Coverage means satisfying filtering and returned fields from index keys without fetching documents; a covered read has zero documents examined. (medium confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) ## How it works - A 2023 Percona forum reporter observed a large $or latency change when adding recurrence to the projection despite apparently equal plans and the same two branch indexes. The thread does not establish the root cause; compare the exact projected operations and execution statistics before attributing such a difference to index choice. (low confidence) — [source](https://forums.percona.com/t/or-query-with-projection-significantly-slower-than-in-mongodb-distribution/24786) - FETCH above IXSCAN can retrieve requested documents even when the index resolves the filter. A filter attached to FETCH identifies additional document-level predicate evaluation; FETCH alone does not prove residual filtering. (medium confidence) — [source](https://www.mongodb.com/docs/manual/tutorial/analyze-query-plan/) ## How-to and procedures - Run find(filter).explain("executionStats") to examine the winning plan and its execution counters. Compare totalKeysExamined and totalDocsExamined with nReturned when assessing the work done for matching results. (medium confidence) — [source](https://www.mongodb.com/docs/manual/tutorial/analyze-query-plan/) ## Problems, failure modes and limitations - For sorted OR queries, distinguish merging ordered scans with SORT_MERGE from a separate SORT. SERVER-24027 illustrates that branch scan directions must provide compatible ordering for the merge; its reported scan-direction defect concerns MongoDB 3.2.3, not a demonstrated current limitation. (medium confidence) — [source](https://jira.mongodb.org/browse/SERVER-24027) ## Comparisons and alternatives - IXSCAN establishes index use, not complete filter coverage or efficiency. FETCH reads collection documents and may apply residual filtering; compare totalKeysExamined and totalDocsExamined with nReturned instead of approving a plan from its stage names. (medium confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - Sorted indexed $or branches can use SORT_MERGE rather than an OR node followed by a blocking SORT. MongoDB's historical SERVER-51120 diagnosis explicitly identifies SORT_MERGE with IXSCAN children for such queries. (medium confidence) — [source](https://jira.mongodb.org/browse/SERVER-51120) - For contained $or queries, common predicates can be distributed into branches. Indraneil Seal illustrates category AND (price OR quantity) and competing category/price and category/quantity indexes; inspect the chosen tree rather than inferring access paths from query syntax. (medium confidence) — [source](https://indraneilseal.ca/2024/05/20/how-mongodb-choose-optimal-path-for-or-operator/) ## Facts and statements - An OR stage with multiple IXSCAN children in inputStages identifies indexed branch union. Read the whole tree: an enclosing FETCH or SUBPLAN can place OR below the root. (high confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - Compound indexes and union can coexist: Moratilla's example merges scans of owner_id/start_date and player_id/start_date indexes. Inspect each branch's indexName and indexBounds to determine which fields constrain that scan. (medium confidence) — [source](https://www.sgmoratilla.com/2022-02-14-mongodb-performance/) - The documented classic plan tree is directly under winningPlan; the slot-based engine exposes the logical tree under winningPlan.queryPlan and also emits slotBasedPlan. A reader must account for this structural difference. (low confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - executionStats describes completed winning-plan execution. allPlansExecution adds partial execution captured during candidate selection; those trial figures are not full executions of every rejected alternative. (low confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - In the documented indexed-OR tree, an OR stage has multiple IXSCAN children in inputStages. Moratilla’s example shows FETCH above OR with separate owner_id/start_date and player_id/start_date compound-index scans; compound indexes and branch union can coexist. (medium confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - Inspect every scan’s indexName, keyPattern, indexBounds and direction. These identify the index and scanned ranges; the presence of an IXSCAN alone does not show that every predicate constrained the scan. (medium confidence) — [source](https://www.sgmoratilla.com/2022-02-14-mongodb-performance/) - totalDocsExamined counts examinations, including filtered-out documents and repeated examinations of the same document; it is neither the result count nor necessarily a unique-document count. (low confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - Read winningPlan for the selected tree and rejectedPlans for alternatives. allPlansExecution adds partial trial statistics, which must not be treated as complete executions of every rejected candidate. (medium confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - Two IXSCAN children do not by themselves identify an OR union. SERVER-106829 records identical scans beneath AND_SORTED in a rejected candidate, illustrating why both the parent stage and candidate location matter. (medium confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/explain-results/) - In Moratilla’s indexed-OR example, start_date has an unbounded descending interval while owner_id or player_id has an equality interval. The compound-index name therefore does not imply that start_date narrows the filter. (low confidence) — [source](https://www.sgmoratilla.com/2022-02-14-mongodb-performance/)