Why is the Order of Evaluation Undefined for SQL User Variables?

The Order of Evaluation Mystery: Why User Variables Break the Rules

In the realm of SQL, the order of evaluation for expressions is usually set in stone. However, there's one exception that can throw a wrench in your queries: user variables. From the output of the infamous query involving user variables (@a), we witness an intriguing result where the order of evaluation is noticeably undefined.

Delving into the depths of the MySQL Manual, we stumble upon a cryptic statement: "the order of evaluation for expressions involving user variables is undefined." What's the reason behind this peculiar behavior?

The answer lies in the enigmatic nature of SQL's evaluation process. As a database optimizer, its primary goal is to efficiently retrieve data and produce accurate results. When dealing with user variables, the optimizer has the freedom to evaluate expressions in any order it deems appropriate.

This flexibility stems from the fact that the SQL standard intentionally leaves the order of evaluation unspecified. Consequently, each database vendor is given the liberty to adopt its own evaluation strategy. The optimizer, acting as the gatekeeper of this strategy, will typically make decisions based on its internal algorithms.

In the absence of a predefined evaluation order, the optimizer can optimize performance by reorganizing the query's execution plan. This optimization aims to minimize the latency associated with variable retrieval and assignment. As a result, the order in which the user variables are evaluated may vary depending on the optimizer's own idiosyncrasies.

In conclusion, the order of evaluation for expressions involving user variables remains undefined due to the discretion granted to database optimizers. This flexibility ensures that databases can adapt to specific hardware configurations and workload patterns, ultimately enhancing query performance and efficiency. However, cautious programmers must remember that relying on a consistent order of evaluation can lead to unpredictable results when working with user variables.

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