質問
I'm trying to use a cursor to clean up temp tables when they are no longer needed. I have a small table which has the names of the temp tables along with an identifier. The cursor is stuck in an infinite loop, but only if I execute certain statements in it. If I just print out the values from the FETCH
, it works perfectly. Here is the code.
DECLARE @id bigint;
DECLARE @table_name varchar(max);
DECLARE st CURSOR LOCAL FAST_FORWARD FOR
SELECT ID, TableName FROM SearchTables WHERE CustomerID IS NULL
OPEN st
FETCH NEXT FROM st INTO @id, @table_name
WHILE @@FETCH_STATUS <> -1
BEGIN
IF(OBJECT_ID(@table_name) IS NOT NULL)
EXEC('DROP TABLE ' + @table_name);
UPDATE SearchTables SET Deleted=1 WHERE ID=@id;
PRINT CAST(@id AS varchar(max)) + ' ' + @table_name;
FETCH NEXT FROM st INTO @id, @table_name;
END
CLOSE st
DEALLOCATE st
If I comment out these lines
IF(OBJECT_ID(@table_name) IS NOT NULL)
EXEC('DROP TABLE ' + @table_name);
UPDATE SearchTables SET Deleted=1 WHERE ID=@id;
PRINT
outputs all of the IDs and table names. If I don't comment them, all I get is the first row over and over until I cancel the query. I also tried changing the IF
line to EXEC('DROP TABLE IF EXISTS ' + @table_name)
but that didn't work either.
解決
Probably you're moving the row when you set Deleted=1
and reading it again with your FAST_FORWARD cursor. Use a STATIC cursor instead, which will iterate a copy of the data, and avoid mutating the data structure you are traversing.
DECLARE st CURSOR LOCAL STATIC FOR . . .
他のヒント
You want WHILE @@FETCH_STATUS = 0
which means continue unless something isn't right.
Using <> -1
means it will continue even if the row fetched was missing, or it's not performing a fetch operation, making it infinite unless you get -1
as a return value, since there are 4 return values for @@FETCH_STATUS
.
0 The FETCH statement was successful.
-1 The FETCH statement failed or the row was beyond the result set.
-2 The row fetched is missing.
-9 The cursor is not performing a fetch operation
.