Cardinality — number of rows a plan step produces or estimates. See 16_QUERY_OPTIMIZATION Lesson 08.
Selectivity — fraction of rows a predicate keeps (matching / total). Low selectivity = doesn’t narrow much.
SARGability — whether a predicate lets the engine seek via an index rather than evaluate per row. See 16_QUERY_OPTIMIZATION Lesson 03.
Execution plan — the physical operation sequence an engine chooses, visible via EXPLAIN.
Predicate — a WHERE/JOIN/HAVING condition evaluating to TRUE, FALSE, or UNKNOWN.
Window — the row set a window function considers for a given row, via PARTITION BY and an optional frame.
Partition (window context) — the group a row belongs to for window function purposes — distinct from table partitioning.
Transaction — a unit the database guarantees executes atomically. See 17_TRANSACTIONS_AND_CONCURRENCY Lesson 01.
Isolation — the ACID property governing visibility of uncommitted concurrent work. See 17_TRANSACTIONS_AND_CONCURRENCY Lesson 02.
Deadlock — a cycle of transactions waiting on each other, resolved by force-rollback of one. See 17_TRANSACTIONS_AND_CONCURRENCY Lesson 04.
Index — an ordered structure letting the engine find rows without a full scan, at write-overhead cost.
Covering index — an index containing every column a query needs, avoiding a table lookup.
Composite index — multi-column index; column order determines what it can efficiently serve (leftmost-prefix rule).
CTE — a named WITH-defined subquery, optionally inlined or materialized depending on engine.
Correlated subquery — references a column from its enclosing query, conceptually re-evaluated per outer row unless rewritten.
Lost update — two transactions read-then-write the same row; the second write silently overwrites the first. See 17_TRANSACTIONS_AND_CONCURRENCY Lesson 02.
MVCC — readers see a consistent snapshot without blocking writers, via multiple row versions. See 17_TRANSACTIONS_AND_CONCURRENCY Lesson 05.
Gaps and islands — identifying consecutive runs/breaks in sequential data, typically via row-number-minus-date (Q014).
Parameter sniffing — a cached plan compiled for one parameter value’s selectivity gets reused for a very different value. See 16_QUERY_OPTIMIZATION Lesson 08.
Relational division — finding entities that satisfy ALL of a required set, typically via nested NOT EXISTS (Q027).