This episode explores the reasons why the seemingly simple SQL syntax "SELECT *" can significantly impact database query performance. Against the backdrop of common misconceptions, the speaker delves into the complexities of row storage in databases, explaining how data is organized into pages and blocks across different database systems. More significantly, the discussion highlights how "SELECT *" prevents index-only scans, leading to costly random reads and increased I/O operations. For instance, the speaker illustrates this with examples involving indexes on specific columns, showing how retrieving all columns necessitates additional lookups, even when only a subset of data is needed. Furthermore, the substantial deserialization costs associated with processing numerous columns, along with the network overhead of transferring large datasets, are detailed. The speaker concludes by emphasizing the importance of requesting only necessary columns to optimize query performance, minimizing network transmission, and reducing overall system load. This underscores the need for developers to understand the underlying mechanics of database operations to write efficient queries.
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