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• Database indexes and their Big-O notation

  - by miket2e

  I'm trying to understand the performance of database indexes in terms of Big-O notation. Without knowing much about it, I would guess that: Querying on a primary key or unique index will give you a O(1) lookup time. Querying on a non-unique index will also give a O(1) time, albeit maybe the '1' is slower than for the unique index (?) Querying on a column without an index will give a O(N) lookup time (full table scan). Is this generally correct ? Will querying on a primary key ever give worse performance than O(1) ? My specific concern is for SQLite, but I'd be interested in knowing to what extent this varies between different databases too.

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• Why is my regex so much slower compiled than interpreted ?

  - by miket2e

  I have a large and complex C# regex that runs OK when interpreted, but is a bit slow. I'm trying to speed this up by setting RegexOptions.Compiled, and this seems to take about 30 seconds for the first time and instantly after that. I'm trying to negate this by compiling the regex to an assembly first, so my app can be as fast as possible. My problem is when the compiling delay takes place: Regex myComplexRegex = new Regex(regexText, RegexOptions.Compiled); MatchCollection matches = myComplexRegex.Matches(searchText); foreach (Match match in matches) // <--- when the one-time long delay kicks in { } This is making compiling to an assembly basically useless, as I still get the delay on the first foreach call. What I want is for all the compiling delay to be done in advance when I compile to the assembly, not when the user runs the app. Where am I going wrong ? (The code I'm using to compile to an assembly is similar to http://www.dijksterhuis.org/regular-expressions-advanced/ , if that's relevant ).

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• How to prevent regex from stopping at the first match of alternatives ?

  - by miket2e

  If I have the string hello world , how can I modify the regex world|wo|w so that it will match all of "world", "wo" and "w" rather than just the single first match of "world" that it comes to ? If this is not possible directly, is there a good workaround ? I'm using C# if it makes a difference: Regex testRegex = new Regex("world|wo|w"); MatchCollection theMatches = testRegex.Matches("hello world"); foreach (Match thisMatch in theMatches) { ... }

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