bcquality/microsoft/knowledge/performance/maintainsqlindex-false-breaks-flowfield-sift.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	maintainsqlindex key sift flowfield sum count table-scan	al	w1	all

MaintainSQLIndex = false on a key disables SIFT for FlowFields that depend on it

Description

MaintainSQLIndex = false on a key tells the platform not to materialize that key as a SQL index. Per the upstream guidance, when a FlowField's source key carries that property, "SIFT cannot function, COUNT/SUM will table-scan." The flag is sometimes set to save write-path cost on a rarely-queried key, but if a CalcFormula aggregates through that exact key, the FlowField loses its index — every CalcFields/CalcSums/list-page filter that triggers it runs without one.

Best Practice

When changing a key property to MaintainSQLIndex = false, find every FlowField whose CalcFormula filters on that key and verify another key covers the same fields. When adding a FlowField whose source table has only a MaintainSQLIndex = false key for its filter columns, add a fully-indexed key (or accept that the FlowField cannot ride SIFT and reshape the design — see flowfield-source-key-needs-sumindexfields.md).

See sample: maintainsqlindex-false-breaks-flowfield-sift.bad.al.

Anti Pattern

A FlowField whose CalcFormula's WHERE columns line up with a key that has MaintainSQLIndex = false. The schema looks correct — the key exists, the SumIndexFields are listed — but at runtime the platform has no SQL index to use, and the aggregation table-scans on every invocation.