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# Bucket reopening and the {metaField, timeField} index requirement

> Source-anchored research for Bucket reopening and the {metaField, timeField} index requirement. Run status: COMPLETED.

Parent: [Bucket fragmentation remediation (re-insert/merge)](https://llms-explorer.com/tree/bucket-fragmentation-remediation-re-insert-merge/) · 3 facets · 8 facts · page: https://llms-explorer.com/tree/bucket-reopening-and-the-metafield-timefield-index-requirement/

## How it works

- The query-based reopening path looks for a bucket with an exact metaField match, a timeField range, bucket count and size limits, and control.closed missing or false. MongoDB's server README says this lookup relies on a {metaField: 1, timeField: 1} index for efficient execution and is not used when that index is absent. (medium confidence) — [source](https://raw.githubusercontent.com/mongodb/mongo/master/src/mongo/db/timeseries/README.md)

## Comparisons and alternatives

- SERVER-95067 lists {timeField: 1}, rather than a compound metaField/timeField index, as the query-based-reopening index when a time-series collection has no metaField. (low confidence) (verified as of 2026-10-23) — [source](https://jira.mongodb.org/browse/SERVER-95067)
- OneUptime's time-series compression article says out-of-order or backfill inserts can reopen closed buckets or create new small buckets, which can degrade compression; it presents this as a workload risk rather than a guaranteed result. (low confidence) (verified as of 2026-04-12) — [source](https://oneuptime.com/blog/post/2026-03-31-mongodb-time-series-compression/view)

## Facts and statements

- MongoDB's server README distinguishes archive-based reopening, which can identify a bucket by its _id, from query-based reopening; the missing compound index disables the query path, not every route for reopening a bucket. (medium confidence) (verified as of 2026-10-20) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/timeseries/README.md)
- MongoDB documents the default {metaField: 1, timeField: 1} compound index as new in 6.3, and the server README says it is created for new collections in 6.3 and later; operators should verify the index on collections created before that default existed. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/timeseries/timeseries-index.md)
- A historical MongoDB issue report says the query-reopening index participated in a lost-first-document defect only with retryable unordered multi-document inserts that concurrently targeted the same bucket on versions 7.0.0–7.0.15 and 8.0.0–8.0.3; it reports fixes in 7.0.16 and 8.0.4, so its workaround to drop the index applies only to those affected versions. (low confidence) (verified as of 2025-01-29) — [source](https://jira.mongodb.org/browse/SERVER-95067)
- For time-series collections with a configured metaField, query-based reopening searches for a disk bucket using exact metadata equality and a timeField range, and relies on the {metaField: 1, timeField: 1} index; if that index is absent, MongoDB does not use query-based reopening. (high confidence) (verified as of 2026-10-20) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/timeseries/README.md)
- Percona's 2021 MongoDB 5.0 article describes automatic timeField indexing and an optional metaField, while MongoDB's manual dates the default metaField/timeField compound index to version 6.3; the default compound-index rule must therefore be scoped by release. (medium confidence) (verified as of 2026-10-14) — [source](https://www.percona.com/blog/mongodb-5-0-time-series-collections/)