What does an empty select do?

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goalng select
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I found the following code in net/http/httptest and wonder what the empty select statement does in Go.

go s.Config.Serve(s.Listener)
if *serve != "" {
  fmt.Fprintln(os.Stderr, "httptest: serving on", s.URL)
  select {}

An empty select{} statement blocks forever. It is similar to an empty for{} statement.

On most (all?) supported Go architectures, the empty select will yield CPU. An empty for-loop won't, i.e. it will "spin" on 100% CPU.

SQL NULL Values - IS NULL and IS NOT NULL, SQL WHERE IS NULL, SELECT WHERE IS NOT NULL, NULL or Value, NULL or Empty. What is NULL and how do I query for NULL values? NULL is a� The empty set is a subset of any set. This is because we form subsets of a set X by selecting (or not selecting) elements from X. One option for a subset is to use no elements at all from X. This gives us the empty set.

On Mac OS X, in Go, for { } will cause the CPU% to max, and the process's STATE will be running

select { }, on the other hand, will not cause the CPU% to max, and the process's STATE will be sleeping

SQL WHERE IS NULL, SELECT WHERE IS NOT NULL, NULL or , Hi, RLT JUNIPA,. Can't really tell from just looking at your formula? Maybe you can try the following. =query(CA!A3:C10,"select Col1 where� select * from DEMO where PARNR = 20 AND TXTFLD IS NULL do not work. We use the .adt file format with the ADsDatarReader. The table contains 20 rows with unset field values and PARNR = 20 for the field TXTFLD. select * from DEMO where PARNR = 20 AND Empty(TXTFLD) provides 4 hits select * from DEMO where PARNR = 20 AND TXTFLD <> 'X' provides 11 hits

The empty select statement just blocks the current goroutine.

As for why you'd do this, here is one reason. This snippet is equivalent

if *serve != "" {
  fmt.Fprintln(os.Stderr, "httptest: serving on", s.URL)
} else {
  go s.Config.Serve(s.Listener)

It's better in that there isn't a wasted goroutine. It's worse in that now there is code repetition. The author optimized for less code repetition over a wasted resource. Note however the permanently block goroutine is trivial to detect and may have zero cost over the duplicating version.

How to filter empty cells with query function, In Oracle, NULLs and empty strings are equivalent when it comes to values stored SELECT id, content, CASE WHEN content IS NULL THEN 1 ELSE 0 END AS Let's do the same thing again, but in PostgreSQL this time. The Null set or Empty set There are some sets that do not contain any element at all. For example, the set of months with 32 days. We call a set with no elements the null or empty set.

How NULL and empty strings are treated in PostgreSQL vs Oracle , All the rows that have data will be hidden. All filtered blank rows together. Now that you have all the empty rows together. you can select all these and delete at one� SQL Query to Select All If Parameter is Empty or NULL In general, when you create a SQL stored procedure or any query that accepts parameters, you might force the User to provide a value for that parameter. It is not the case in real-time, so you have to allow NULL values and empty strings.

How to Delete Empty Rows in Google Sheets (3 Easy Ways , Three of the records have no Body assignment (and therefore are empty sets), an arbitrary expression can be used, such as in SELECT and HAVING clauses. SQL SELECT statement is used to query or retrieve data from a table in the database. A query may retrieve information from specified columns or from all of the columns in the table. To create a simple SQL SELECT Statement, you must specify the column(s) name and the table name.

IS_EMPTY and IS_NOT_EMPTY functions, The primary table key of a standard table can be empty. An empty Explicitly using an inline declaration INTO TABLE @DATA(itab) in the statement SELECT. LINQ Query Issues when trying to select from empty collection. Ask Question Asked 6 years, 7 months ago. Active 6 years, 7 months ago. on an empty var.

  • That was my guess. However I still find it semantically awkward, because select suggests a condition and not a loop.
  • AFAIK an empty for would be intensive on the cpu (at least that's how empty infinite loops work in other languages). While in OP's example it's likely that select{} is used to prevent the main goroutine from exiting.
  • In CSP-speak, the empty select is like STOP, a process that never proceeds. This is akin to a self-deadlock and I imagine it doesn't get used all that often. However, the question proposed an interesting and genuine use-case for this pattern. An empty for{} loop is a self-livelock, different because it consumes cpu resources.
  • It appears that it's idiomatic Go to use select{} to block the main goroutine which yields the CPU. Since the main goroutine hasn't returned, this leaves the process around and one can analyze the behavior of the other goroutines.