大约有 40,000 项符合查询结果（耗时：0.0371秒） [XML]
Storing time-series data, relational or non?
...evation.
Correction
It is not "filtering out the un-needed data"; it is selecting only the needed data. Yes, of course, if you have an Index to support the columns identified in the WHERE clause, it is very fast, and the query does not depend on the size of the table (grabbing 1,000 rows from a ...
MySQL, update multiple tables with one query
...'blah';
To see what this is going to update, you can convert this into a select statement, e.g.:
SELECT t2.t1_id, t2.t3_id, t1.a, t2.b, t2.c AS t2_c, t3.c AS t3_c
FROM t1
INNER JOIN t2 ON t2.t1_id = t1.id
INNER JOIN t3 ON t2.t3_id = t3.id
WHERE t1.a = 'blah';
An example using the same tables as...
#1071 - Specified key was too long; max key length is 1000 bytes
... the best prefix length for a given column? Here's a method to find out:
SELECT
ROUND(SUM(LENGTH(`menu_link`)<10)*100/COUNT(`menu_link`),2) AS pct_length_10,
ROUND(SUM(LENGTH(`menu_link`)<20)*100/COUNT(`menu_link`),2) AS pct_length_20,
ROUND(SUM(LENGTH(`menu_link`)<50)*100/COUNT(`menu_...
Apply multiple functions to multiple groupby columns
... group. When aggregating, g will be a Series. Passing g.index to df.ix[] selects the current group from df. I then test if column C is less than 0.5. The returned boolean series is passed to g[] which selects only those rows meeting the criteria.
In [95]: cust = lambda g: g[df.loc[g.index]['C']...
javac not working in windows command prompt
...anced" tab ⇒ Environment Variables
In "System Variables", scroll down to select "PATH" ⇒ Edit
(( now read the following 3 times before proceeding, THERE IS NO UNDO ))
In "Variable value" field, INSERT "c:\Program Files\Java\jdk1.8.0_xx\bin" (Replace xx with the upgrade number and VERIFY that ...
Haskell: Lists, Arrays, Vectors, Sequences
...st. The best example of lists screwing up performance tends to come from [Char] which the prelude has aliased as String. Char lists are convient, but tend to run on the order of 20 times slower than C strings, so feel free to use Data.Text or the very fast Data.ByteString. I'm sure there are othe...
Hidden features of Perl?
...
One of my favourite features in Perl is using the boolean || operator to select between a set of choices.
$x = $a || $b;
# $x = $a, if $a is true.
# $x = $b, otherwise
This means one can write:
$x = $a || $b || $c || 0;
to take the first true value from $a, $b, and $c, or a default of 0...
Why don't structs support inheritance?
...hat short[] can't be converted to int[] (short of conversion code, like 'a.Select(x => (int) x).ToArray()'). If the runtime disallowed the cast from Base to Derived, it would be a "wart", as that IS allowed for reference types. So we have two different "warts" possible -- forbid struct inherita...
What's the difference between --general-numeric-sort and --numeric-sort options in gnu sort
...0e-34
and 10e100. The LC_NUMERIC locale
determines the decimal-point
character. Do not report overflow,
underflow, or conversion errors. Use
the following collating sequence:
Lines that do not start with numbers
(all considered to be equal). NaNs
(“Not a Number” values, in IEEE...
Use space as a delimiter with cut command
...
can you tell cut to use any number of a certain character as the delimiter, like in RegEx? e.g. any number of spaces, e.g. \s+
– amphibient
Nov 1 '12 at 15:42
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