I have a very simple SQL query: SELECT COUNT(DISTINCT x) FROM table; My table has about 1.5 million rows. This query is running pretty slowly; it takes about 7.5s, compared to SELECT COUNT(x) FROM table; which takes about 435ms. Is there any way to change my query to improve performance? I’ve tried grouping and doing … Read more
Background: While optimizing some Pascal code with embedded assembly language, I noticed an unnecessary MOV instruction, and removed it. To my surprise, removing the un-necessary instruction caused my program to slow down. I found that adding arbitrary, useless MOV instructions increased performance even further. The effect is erratic, and changes based on execution order: the … Read more
I believe there’s a way to find the kth largest element in an unsorted array of length n in O(n). Or perhaps it’s “expected” O(n) or something. How can we do this? 33 Answers 33 This is called finding the k-th order statistic. There’s a very simple randomized algorithm (called quickselect) taking O(n) average time, … Read more
I’ve Googled and poked around the Go website, but I can’t find an explanation for Go’s extraordinary build times. Are they products of the language features (or lack thereof), a highly optimized compiler, or something else? I’m not trying to promote Go; I’m just curious. 11 Answers 11
Is there a performance hit if we use a loop instead of recursion or vice versa in algorithms where both can serve the same purpose? Eg: Check if the given string is a palindrome. I have seen many programmers using recursion as a means to show off when a simple iteration algorithm can fit the … Read more
Let’s say I specify an outputText component like this: <h:outputText value=”#{ManagedBean.someProperty}”/> If I print a log message when the getter for someProperty is called and load the page, it is trivial to notice that the getter is being called more than once per request (twice or three times is what happened in my case): DEBUG … Read more
The –depth 1 option in git clone: Create a shallow clone with a history truncated to the specified number of revisions. A shallow repository has a number of limitations (you cannot clone or fetch from it, nor push from nor into it), but is adequate if you are only interested in the recent history of … Read more
Haskell (with the GHC compiler) is a lot faster than you’d expect. Used correctly, it can get close-ish to low-level languages. (A favorite thing for Haskellers to do is to try and get within 5% of C (or even beat it, but that means you are using an inefficient C program, since GHC compiles Haskell … Read more
Given two inclusive ranges [x1:x2] and [y1:y2], where x1 ≤ x2 and y1 ≤ y2, what is the most efficient way to test whether there is any overlap of the two ranges? A simple implementation is as follows: bool testOverlap(int x1, int x2, int y1, int y2) { return (x1 >= y1 && x1 <= … Read more
I know that recursion is sometimes a lot cleaner than looping, and I’m not asking anything about when I should use recursion over iteration, I know there are lots of questions about that already. What I’m asking is, is recursion ever faster than a loop? To me it seems like, you would always be able … Read more
