Filtrare con WHERE
Migliorare le prestazioni delle query in PostgreSQL
Amy McCarty
Instructor
Limita i dati
| Order | Clause | Purpose |
|---|---|---|
| 1 | FROM | indica le tabelle |
| 2 | WHERE | filtra/limita i record |
- Avviene presto
- Meno record
EXPLAIN
EXPLAIN
SELECT * FROM phones
Seq Scan on phones (cost = 0.00..22.7
,rows=1270
,width=36)
- Numero di passaggi di esecuzione
Query planner
EXPLAIN con WHERE
EXPLAIN
SELECT * FROM phones
WHERE phone_code = 235
Seq Scan on phones (cost = 0.00..25.8
,rows=6,width=636)
Filter: (phone_code=235)
Query planner
Buono: filtra valori simili con LIKE OR
| country | phone_code | reliability |
|---|---|---|
| Chad | 235 | medium |
| China | 86 | high |
| Costa Rica | 506 | high |
| India | 91 | medium |
| Indonesia | 62 | medium |
| Iraq | 964 | low |
EXPLAIN
SELECT * FROM phones
WHERE country LIKE 'Ch%'
OR country LIKE 'In%'
Seq Scan on phones (cost = 0.00..29.05
,rows=13,width=36)
Filter: ((country~~'Ch%'::text)
OR(country~~'In%'::text))
Meglio: filtra valori simili con LIKE ANY
| country | phone_code | reliability |
|---|---|---|
| Chad | 235 | medium |
| China | 86 | high |
| Costa Rica | 506 | high |
| India | 91 | medium |
| Indonesia | 62 | medium |
| Iraq | 964 | low |
EXPLAIN
SELECT * FROM phones
WHERE country
LIKE ANY(ARRAY['Ch%','In%'])
Seq Scan on phones (cost = 0.00..25.88
,rows=13,width=36)
Filter: ((country~~ANY('{Ch%,In%}'
::text[]))
Buono: filtra valori esatti con OR
| country | phone_code | reliability |
|---|---|---|
| Chad | 235 | medium |
| China | 86 | high |
| Costa Rica | 506 | high |
| India | 91 | medium |
| Indonesia | 62 | medium |
| Iraq | 964 | low |
EXPLAIN
SELECT * FROM phones
WHERE country = 'Chad'
OR country = 'China'
Seq Scan on phones (cost = 0.00..29.05
,rows=13,width=36)
Filter: ((country='Chad'::text)
OR(country='China'::text))
Meglio: filtra valori esatti con IN
| country | phone_code | reliability |
|---|---|---|
| Chad | 235 | medium |
| China | 86 | high |
| Costa Rica | 506 | high |
| India | 91 | medium |
| Indonesia | 62 | medium |
| Iraq | 964 | low |
EXPLAIN
SELECT * FROM phones
WHERE country IN ('Chad','China')
Seq Scan on phones (cost = 0.00..25.88
,rows=13,width=36)
Filter: ((country=ANY('{Chad,China}'
::text[]))
Ottimo: filtra numeri
| country | phone_code | reliability |
|---|---|---|
| Chad | 235 | medium |
| China | 86 | high |
| Costa Rica | 506 | high |
| India | 91 | medium |
| Indonesia | 62 | medium |
| Iraq | 964 | low |
EXPLAIN
SELECT *
FROM phones
WHERE phone_code IN (235,86)
Seq Scan on phones (cost = 0.00..25.88
,rows=13,width=36)
Filter: (phone_code=ANY('{235,86}'
::integer[]))
Riepilogo: i migliori filtri WHERE
Vantaggi dei numeri
- Più breve
- Meno spazio
- Migliora le prestazioni
Riepilogo: i migliori filtri WHERE
Vantaggi dei numeri
- Più breve
- Meno spazio
- Migliora le prestazioni
| Buono | Meglio |
|---|---|
| Testo | Numerico |
| OR | IN, ARRAY |
Passons à la pratique !
Migliorare le prestazioni delle query in PostgreSQL
