sql - ORACLE NOT NULL IN WHERE CLAUSE WITH NVL OPERATION -


i want know query faster.

ex:1

select comm emp comm not null;

ex:2

select comm emp nvl(comm,'0') not null;

please consider emp table contains hundreds of thousands of records.

instead of asking hypothetical question should run benchmarks on actual data. here observations:

your queries different things. first query selects records comm not null. second query selects records.

in both cases 2 full table scans: " lakhs of records." makes no difference. however, given have same projection , no sort clause execution time same.

except:

  1. the time render output different, because second result set may larger first.
  2. if there index on emp(comm) first result use full fast index scan, because you're selecting comm there's no need touch table. index read faster because normal oracle indexes don't index nulls.
  3. even if there's function-based index on emp(nvl(comm)) second query still execute full table scan, because select comm in projection query still needs read table.

Comments

Post a Comment

Popular posts from this blog

javascript - Clear button on addentry page doesn't work -

this addentry blog page have made scratch , there underlying problems code particularly clear button supposed clear text entered when button clicked on doesn't work have saved html file problem. also 1 other problem submit button works doesn't pass of text index file have written php , reason whenever click on submit button redirects me index page without posting entry have made. <!doctype html> <html> <head> <script> function message() { alert("clear form?"); } </script> <style> body {background-color: beige;} h1 {color: black;} p {color: black;} textarea { width: 50%; height: 150px; padding: 12px 20px; box-sizing: border-box; border: 2px solid #ccc; border-radius: 4px; background-color: white; font-size: 16px; resize: none; } div { padding-left: 20px; } </style> </head> <meta charset="utf-8"> <link rel="stylesheet" href=".
Read more

c# - Selenium Authentication Popup preventing driver close or quit -

i have selenium automated task setup login website retrieve information. it's basic in automation. logging in , clicking 2 links. running issue proving problem. the site in question uses authentication required popup not javascript popup rather regular windows auth popup. have read isn't handled out of box selenium. i using " http://username:password@url.com " site every , site has issues on end prompt me again username/password randomly. have enter username/password twice in row when navigating there hand. since task loops continuously few hours picking needed info figured use webdriverwait verify element present on page. if authentication popup appears, element isn't present. no need care run, start over. the problem having when authentication popup displayed , hit timeout limit want close driver/browser window can start task on again. however, driver.close() , driver.quit() not doing anything. browser remains open , unloaded page still there login p
Read more

tensorflow when input_data MNIST_data , zlib.error: Error -3 while decompressing: invalid block type -

i'm geting started tensorflow,followed tutorials,but cannot download mnist data. import tensorflow tf tensorflow.examples.tutorials.mnist import input_data mnist=input_data.read_data_sets("mnist_data/",one_hot=true) print("download done!") error information: extracting mnist_data/train-images-idx3-ubyte.gz traceback (most recent call last): file "/home/sj/program/tesorflow_try.py", line 6, in <module> mnist=input_data.read_data_sets("mnist_data/",one_hot=true) file "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/learn/python/learn/datasets/mnist.py", line 213, in read_data_sets train_images = extract_images(f) file "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/learn/python/learn/datasets/mnist.py", line 60, in extract_images buf = bytestream.read(rows * cols * num_images) file "/usr/lib/python2.7/gzip.py", line 268, in read self._read(readsize) file
Read more