Sequential scan
A sequential scan is a search or query operation that examines items in a table or collection in sequence to identify those matching a condition, rather than locating them through an index.
Just Use Postgres!
A sequential scan, or full table scan, examines every row of a table to find rows matching a query rather than locating them through an index.
- How it is used
- Uses full-row inspection to answer queries when no suitable index exists or scanning is judged efficient.
- How deep it goes
- Explains linear behavior, filtering, row counts, parallel execution, and planner tradeoffs without detailing scan implementation internals.
- Examples
- Demonstrates scans on player wins, score ranges, champion titles, JSON fields, time-series measurements, and message queues.
- What you can do after
- Diagnose/debug with EXPLAIN by identifying Seq Scan nodes, filters, rejected rows, timing, and possible indexing opportunities.
PostgreSQL Mistakes and How to Avoid Them
A full table scan is a query execution method that examines rows throughout a table rather than locating them through a selective index lookup.
- How it is used
- Diagnoses slow PostgreSQL queries whose plans scan entire tables instead of using selective indexes.
- How deep it goes
- Explains plan inspection, predicate matching, partial indexes, foreign-key indexing, and expression-predicate workarounds without exhaustive index-type detail.
- Examples
- Uses open-ticket counts, timestamped payments, and invoice lookups by customer to compare scan and index-backed plans.
- What you can do after
- Diagnose/debug with it by reading PostgreSQL plans and selecting predicate-aligned indexing or query rewrites.
Read how PostgreSQL Mistakes and How to Avoid Them explains sequential scan →
MongoDB 8.0 in Action, Third Edition
A collection scan is query execution that examines documents throughout a collection to find matches without relying on an applicable index.
- How it is used
- Explains how MongoDB retrieves matches when no applicable index supports a query.
- How deep it goes
- Explains scan behavior, index trade-offs, $or requirements, and selectivity, but does not detail scan internals.
- Examples
- Uses server-log queries and an unindexed movie-rating $or clause to demonstrate collection-scan behavior.
- What you can do after
- Diagnose collection scans with explain and improve eligible $or queries by adding supporting indexes.
Read how MongoDB 8.0 in Action, Third Edition explains sequential scan →
Related concepts
- index scancontrasts with · An index scan is a database query operation that traverses an index to locate or retrieve records matching query conditions.
- indexingcontrasts with · Indexing is the organization or use of positions, keys, or lookup structures to identify and retrieve selected data or elements.
- membership operatorused by · A membership operator is a language construct that tests whether a specified value or property is present in a supported collection, range, or object.
- composite indexcontrasts with · A composite index is a database index whose key contains multiple fields or columns, ordered according to the index definition.
- listuses · A list is an ordered collection of elements arranged in a sequence.
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