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The queries that cause a large number of logical reads usually acquire locks on a correspondingly large set of data. Even reading (as opposed to writing) may require shared locks on all the data, depending on the isolation level. These queries block all other queries requesting this data (or part of the data) for the purposes of modifying it, not for reading it. Since these queries are inherently costly and require a long time to execute, they block other queries for an extended period of time. The blocked queries then cause blocks on further queries, introducing a chain of blocking in the database. (Chapter 13 covers lock modes.)


Cover of SQL Server Query Performance Tuning,Fourth Edition


effect on lots of logical reads