Database Indexes chay' Qu'

"index chel" database advice law' 'ej misapplied law' - slow queries Daq index nuq Qu' Sov pagh. index pagh pagh choH reH. index second separately-sorted copy data slice, table rap synchronize: write cost 'ej storage read speed trade (access patterns specific).

index pagh: full scan

column index pagh table, column filter query vaD, unordered pile rows. email = 'user@example.com' rows tu' vaj database Hoch row legh.

SELECT * FROM users WHERE email = 'user@example.com';
-- Seq Scan on users (cost=0.00..18334.00 rows=1 width=120)
--   Filter: (email = 'user@example.com'::text)

few hundred rows instant. ten million rows: lookup Hoch ten million comparisons - table grow vaj linear worse.

index nuq

indexes law' (Postgres default, MySQL InnoDB, SQL Server) B-trees: balanced tree structure, node Hoch sorted key values 'ej pointers (child nodes pagh leaf level Daq real row).

CREATE INDEX idx_users_email ON users (email);

email sorted tree chenmoH. 'user@example.com' tu': tree walk down - node values compare, pointer follow, repeat (table scan pagh). million rows B-tree ~20 comparisons (million pagh).

EXPLAIN SELECT * FROM users WHERE email = 'user@example.com';
-- Index Scan using idx_users_email on users (cost=0.42..8.44 rows=1 width=120)
--   Index Cond: (email = 'user@example.com'::text)

EXPLAIN output cost 18334 8.44 Daq pIm - indexing pitch wa' line.

index columns covers neH Qu'

email index created_at filter query vaD Qu'be'. multi-column index leftmost column filter Daq pagh vaj Qu'be'.

CREATE INDEX idx_orders_user_status ON orders (user_id, status);

-- uses the index: user_id is the leftmost column
SELECT * FROM orders WHERE user_id = 42;
SELECT * FROM orders WHERE user_id = 42 AND status = 'shipped';

-- does NOT use this index: status alone isn't the leftmost column
SELECT * FROM orders WHERE status = 'shipped';

multi-column index phone book rap: last name, first name. "Smith" jump, "Smith, John" - 'ach "John" Hoch tu' (last name Hoch legh pagh) laHbe'. composite index column order cosmetic pagh: queries nuq serve decide.

indexes free pagh

index Hoch INSERT, UPDATE, DELETE indexed column touch vaj update - write work law', storage law'. eight indexes table insert Hoch eight times cost. blindly column Hoch index Qagh: writes slow, database bloat (unused indexes).

unused indexes tu':

-- Postgres: indexes with zero scans since the last stats reset
SELECT relname AS table, indexrelname AS index, idx_scan
FROM pg_stat_user_indexes
WHERE idx_scan = 0;

unused index cost neH - benefit pagh, write storage tax pay.

LIKE 'ej index ignore

SELECT * FROM users WHERE email LIKE '%example.com';  -- can't use a standard B-tree index
SELECT * FROM users WHERE email LIKE 'user@%';         -- can

B-tree index sorted: prefix ('user@%') efficient tu' (sorted order Daq jump, forward laD) 'ach suffix ('%example.com') pagh - sort order Daq nuqDaq laH, full scan. suffix pagh substring search real requirement vaj trigram indexes (Postgres pg_trgm) pagh dedicated search engine. B-tree Qagh tool.

EXPLAIN index qaSpa'

"index chel" instinct step skip: query plan wa'DIch legh.

EXPLAIN ANALYZE
SELECT * FROM orders WHERE user_id = 42 AND status = 'shipped';

EXPLAIN ANALYZE query Qu' 'ej actual report: Seq Scan vs Index Scan, rows examined vs returned, real timing. query index lo' 'ach slow vaj fix: pIm index (more columns cover), smarter query (indexed column function call index defeat), pagh queries (full year reporting) rows law' scan accept.

yaj

index "row Hoch legh" "sorted tree walk" choH, 'ach columns cover 'ej access patterns support neH - blanket performance switch pagh. queries actually run vaD indexes chel, column order filter rap, EXPLAIN database lo' confirm.