Reading the Signs in EXPLAIN: When Queries Ignore Your Indexes
Even with an index in place, seeing a full scan in EXPLAIN is common. Four patterns sever the link between a query and its index: function-wrapped columns, type mismatches, leading wildcards in LIKE, and low selectivity. This post is a decision memo that walks through each pattern with real examples of how it distorts the execution plan, and lays out a procedure for narrowing down and fixing the cause, from reading EXPLAIN to confirming the root cause.
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