SQL
Realistic Scenarios: Analysis and Optimisation
Six end to end problems of the kind real work produces - a sales report, a retention table, a data quality audit, a duplicate cleanup and two optimisa...
#Query Optimisation
Execution plans and query rewriting.
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SQL
Query Optimisation and Pagination Performance
A practical checklist for making a slow query fast, and the specific fix for deep pagination that no amount of indexing solves.
SQL
Scans, Seeks, Join Strategies and Sorting
What the engine actually does: full scans versus index seeks, nested loop and hash joins, and why a sort or a temporary table appears in your plan.
SQL
How a Query Executes and Reading EXPLAIN
What happens between typing a query and getting rows, and how to read an execution plan - the single most useful performance skill in SQL.
SQL
When Indexes Help and When They Hurt
Indexes are not free. Learn the cases where the optimiser correctly ignores them, how to find unused and duplicate indexes, and how to maintain them.