Is SQL's `IN` Operator Always Efficient? A Performance Deep Dive

Performance impact of SQL IN operators: in-depth analysis

When building queries using the SQL IN operator, you must consider several factors that may affect performance.

IN Internal rewriting of clause

Databases often internally rewrite IN clauses to use the OR joiner. For example, col IN ('a','b','c') will be converted to: (COL = 'a') OR (COL = 'b') OR (COL = 'c'). If an index exists on the col column, the execution plans of the two queries are usually equivalent.

Duplicate parsing of dynamic queries

When using IN or OR with a variable number of parameters, the database must reparse the query and rebuild the execution plan each time the parameters change. This is a costly process. To ensure optimal performance, the use of bind variables is highly recommended. By using bind variables, the database can cache execution plans for queries with the same query text.

Query complexity limit

Most databases limit the complexity of the queries they can execute, including the number of logical connectors in a predicate. While a few dozen values ​​in the IN clause are unlikely to reach this limit, hundreds or thousands of values ​​may cause the database to cancel the query.

Parallelization Limitations

Queries containing predicates of IN or OR may not always be optimally rewritten for parallel execution. In some cases, parallelization optimizations may not be applied. Alternatively, when feasible, queries using the UNION ALL operator are easier to parallelize and should be preferred over logical join operators.

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