DDevelopment & Design

Indeksy na kolumnach z wartościami nullowymi cz. 2

Jest jeszcze jeden sposób indeksowania, który pozwala nam na bardzo wygodne korzystanie z indeksów i wyszukiwanie wartości nullowych. Mianowicie indeks jest prostym indeksem składającym się z dwóch kolumn – tej po której chcemy wyszukiwać i wartości stałej. Np: 

create index kolumna_IDX on tabela (kolumna, TO_DATE('4000-01-01', 'yyyy-mm-dd'));

i wtedy możemy pisać zapytania: 

select * from tabela where kolumna is null

i indeks zadziała! Zrobiłam też porównanie dla tabeli, gdzie stosunek ilości nullowych wierszy do wszystkich w tabeli wynosi: 1.002/131.002 I założyłam indeks 

create index AA2_IDX on AAAAA2_TEST (NVL(MODIFY_DATE,TO_DATE('4000-01-01', 'yyyy-mm-dd')))

Koszt zapytania: 

select COUNT(*) from aaaaa2_test WHERE NVL(modify_DATE,TO_DATE('4000-01-01', 'syyyy-mm-dd')) = TO_DATE('4000-01-01', 'syyyy-mm-dd')

wyniósł 38 zaś dla indeksu: 

create index AA_IDX on AAAAA2_TEST (FROM_DATE, TO_DATE('4000-01-01', 'yyyy-mm-dd'))

i zapytania: 

select COUNT(*) from aaaaa2_test WHERE FROM_DATE IS NULL

wyniósł 28. Dla większej ilości nullowych rekordów tzn. 11002/131.002 stosunek kosztów wyniósł 43/28, a więc znów zdecydowanie na korzyść indeksu dwukolumnowego.

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From time to time I have a need to do some fixes in my json data. In a world of flat files I do this with grep/sed/awk tool chain. How to handle it for JSON? Searching for a solution I came across the JSONPath. It quite mature tool (from 2007) but I haven't hear about it so I decided to share my experience with others.

First of all you can try it without pain online: http://jsonpath.curiousconcept.com/. Full syntax is described at http://goessner.net/articles/JsonPath/



But also you can download python binding and run it from command line:
$ sudo apt-get install python-jsonpath-rw
$ sudo apt-get install python-setuptools
$ sudo easy_install -U jsonpath

After that you can use inside python or with simple cli wrapper:
#!/usr/bin/python
import sys, json, jsonpath

path = sys.argv[1]

result = jsonpath.jsonpath(json.load(sys.stdin), path)
print json.dumps(result, indent=2)

… you can use it in your shell e.g. for json:
{
"store": {
"book": [
      {
"category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
      },
      {
"category": "fiction",
"author": "Evelyn Waugh",
"title": "Sword of Honour",
"price": 12.99
      },
      {
"category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
      },
      {
"category": "fiction",
"author": "J. R. R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
      }
    ],
"bicycle": {
"color": "red",
"price": 19.95
    }
  }
}

You can print only book nodes with price lower than 10 by:
$ jsonpath '$..book[?(@.price

Result:
[
  {
"category": "reference",
"price": 8.95,
"title": "Sayings of the Century",
"author": "Nigel Rees"
  },
  {
"category": "fiction",
"price": 8.99,
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"author": "Herman Melville"
  }
]

Have a nice JSON hacking!From time to time I have a need to do some fixes in my json data. In a world of flat files I do this with grep/sed/awk tool chain. How to handle it for JSON? Searching for a solution I came across the JSONPath. It quite mature tool (from 2007) but I haven't hear about it so I decided to share my experience with others.