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MongoDBIntermediateThe 100 MB blocking-sort limit means your index doesn't provide the sort order. Build compound indexes with the ESR rule, order pipeline stages correctly, and know when allowDiskUse is fine.
#indexing#nosql#performance
Oracle DatabaseIntermediateYour long query needed undo that was already overwritten. How Oracle read consistency uses undo, how to read V$UNDOSTAT, and why committing inside a loop causes it.
#configuration#mvcc#troubleshooting
MySQLIntermediateStale reads and 2 a.m. lag spikes usually come down to single-threaded apply, tables without primary keys and giant transactions. Diagnose and fix each one.
#performance#replication#troubleshooting
SQL ServerIntermediateNOLOCK doesn't just risk dirty reads. It can skip rows, count rows twice and fail with error 601. Read Committed Snapshot Isolation removes the blocking without the wrong answers.
#concurrency#locking#transactions
GeneralIntermediateSelect-then-insert breaks under concurrency. Use a unique constraint plus one atomic statement: ON CONFLICT, ON DUPLICATE KEY UPDATE, or UPDLOCK/SERIALIZABLE in SQL Server.
#concurrency#schema-design#transactions
PostgreSQLIntermediateMVCC means Postgres doesn't store a row count. Make exact counts cheaper, get instant estimates from the planner, and keep exact counters when you really need them.
#internals#mvcc#performance
SQL ServerIntermediateExecution plans tell you exactly how SQL Server ran your query. Learn which operators matter, how to spot bad estimates and what the warnings really mean.
#execution-plans#performance#t-sql
SQL ServerIntermediateWhat each isolation level protects you from, what it costs, and why so many teams switch on READ_COMMITTED_SNAPSHOT.
#concurrency#locking#transactions
PostgreSQLIntermediateLearn to read PostgreSQL query plans node by node — estimates vs. actuals, loops, buffers — and turn them into index and query fixes.
#execution-plans#indexing#performance
MySQLIntermediateIn MySQL's InnoDB, the table is the primary key index. That single fact shapes insert speed, index size and how you should pick primary keys.
#indexing#internals#schema-design
SQLiteIntermediateSQLite can serve real production traffic. Here's how WAL mode changes concurrency, which PRAGMAs to set, and where the limits are.
#concurrency#internals#performance
