bcquality/microsoft/knowledge/performance/prefer-dictionary-over-temporary-table-for-lookups.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	dictionary temporary-table lookup o-of-1 key-lookup	al	w1	all

Prefer a Dictionary over a temporary table for pure lookups

Description

An AL Dictionary directly models an unordered unique key-to-value collection. A temporary table models records and supports keys, filters, validation, and ordered iteration in Business Central Server memory. For a pure lookup map, the dictionary avoids repeatedly configuring and searching a temporary record and makes the intended access pattern explicit.

Best Practice

Use Dictionary of [Key, Value] when the operation is add-or-replace, contains-key, and get-value by one supported key type. Use a temporary table when the value is a record, or when the code needs filters, ordered iteration, multiple fields, multiple keys, or table behavior. Both structures consume service-tier memory and still need volume analysis.

Anti Pattern

A temporary record used only through SetRange(KeyField, X); FindFirst() to retrieve one scalar value, with no record semantics that justify the table. The opposite mistake is replacing a temporary table that needs filtering or ordered iteration with a dictionary.