Oracle SQL

Compare conventional and direct-path inserts of 500,000 rows by time, redo, and undo, and see when the APPEND hint really pays off.

Measure hard parses, time, and shared pool memory for literal SQL and bind variables, and see what CURSOR_SHARING = FORCE changes.

Watch bind peeking and adaptive cursor sharing on a skewed column, read the child cursors in V$SQL, and see when the first plan sticks.

Write optimizer hints that work, find the ones Oracle ignores with the Hint Report, and see why forcing a plan can make queries slower.

See how correlated columns fool the optimizer, create a column group with DBMS_STATS, and make sure the optimizer really uses it.

See how stale statistics mislead the optimizer on new data, find stale and missing statistics, and gather them again with DBMS_STATS.

See how skewed data fools the optimizer without histograms, how SIZE AUTO creates them, and how frequency and hybrid histograms fix plans.

Read index usage from DBA_INDEX_USAGE, see why sampled tracking can miss rare uses, and remove unused indexes safely.

Six common reasons Oracle does a full scan despite an index, each shown in a real plan with the change that makes the index work.

Rank SQL statements by total and per-execution time with V$SQLSTATS, read child cursors in V$SQL, and show the top statement's plan.

See nested loops, hash joins and sort-merge joins in real Oracle plans, learn when each is chosen, and compare their cost on one query.

See every common Oracle access path in a real plan, learn when the optimizer picks it, and why a full scan is sometimes faster than an index.

Get a plan with EXPLAIN PLAN and DBMS_XPLAN, read its operations in the right order, and compare the estimates with actual rows.

See why committing inside a FOR UPDATE cursor loop breaks the next fetch, and how to commit after the loop or chunk the work.

Reproduce the lock conflict with two sessions, and fix it with WAIT, SKIP LOCKED, DDL_LOCK_TIMEOUT, or by ending the blocking transaction.

Find which system privileges the session really has, and grant the missing one, directly when views or PL/SQL need it.

Understand why CURRVAL belongs to the session, call NEXTVAL first, and capture generated keys with RETURNING INTO.

Change the data type of a column that already holds data, by adding a new column, copying, dropping, and renaming.