What does it mean for a SQL predicate to be SARGable, and why does it matter?

Explain SARGable predicates and index usage

Writes predicates the optimizer can use as index search arguments, then proves the effect through plans and reads while balancing computed columns, composite indexes, and write overhead.

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A SARGable predicate exposes a search argument the optimizer can use to seek into an index instead of evaluating every row. For a date column, CreatedAt >= @start AND CreatedAt < @end can use a range seek, while CAST(CreatedAt AS date) = @day often applies work to each row and leads to a scan. I also watch leading-wildcard searches and implicit conversions on the indexed column. SARGability improves the available plan; it does not guarantee the optimizer will choose a seek when a scan is cheaper.

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Index usage, predicate shape, functions on columns, date ranges, LIKE patterns, and query rewrites.

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Why this question matters
A suitable index cannot help efficiently when the predicate hides the searchable column behind a function, incompatible conversion, or unsupported pattern. SARGability connects expression shape to the access path the optimizer can choose.
A SARGable predicate exposes a search argument the optimizer can use to seek into an index instead of evaluating every row. For a date column, CreatedAt >= @start AND CreatedAt < @end can use a range seek, while CAST(CreatedAt AS date) = @day often applies work to each row and leads to a scan. I also watch leading-wildcard searches and implicit conversions on the indexed column. SARGability improves the available plan; it does not guarantee the optimizer will choose a seek when a scan is cheaper.
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Calling every index scan evidence of a bad plan.
Rewriting a predicate for speed without proving equivalent date or null semantics.
Allowing parameter types to force conversion of the indexed column.
Adding a computed index without accounting for writes and storage.

What does it mean for a SQL predicate to be SARGable, and why does it matter?

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