I have a table say, TestTable which has columns StartDate, 1stDeliveryDate, DeliveryFrequency, HolidayCalenderId(which links to other table calender).
Now i have to add a column FinalDeliveryDate which needs some calulation using the other fields mentioned above.I have managed to write a Function which does the calculations and updates the field. The final query to update looks something like,
Update TestTable
Set FinalDeliveryDate = dbo.GetCalculatedFinalDeliveryDate(StartDate, 1stDeliveryDate, HolidayCalenderId)
WHere dbo.GetCalculatedFinalDeliveryDate(StartDate, 1stDeliveryDate, HolidayCalenderId) >= GETDATE
This example above is just to explain the scenario how the function is used. Now the size of TestTable is increasing rapidly as the business is growing. I am worried about performance as the query needs to run quite frequently and on all the rows in the test table. Just to clarify, I am a developer and not a DBA, but after researching about functions, I know its best to avoid them in Select and Where clause. If possible can someone point me in the right direction for how to remove this function and do a set operation to update the rows.. I am not able to think of any other way to replace this fucntion..
Related
How to store, retrieve and update a sequence number in a single row of a table with a schema like:
ID (int)
LookUp(varchar)
SeqNum(int) --business logic dictates the SeqNum is constrained to a particular range, say 1300 to 7600
To me this looks like the clickercounter guy at a ball park using a clicker to ticks one off for each person that goes by. I want each person to have a unique number. I want multiple clickercounter people to use the same clicker and I don't want any missed values.
So far my approaches have either resulted in a deadlock condition leaving the table inaccessible or me scratching my head wondering about how to structure a stored procedure that calls a stored procedure that has a transaction to lock the record, read it, update it, commit the transaction, and unlock the record
In pseudo code I tried something like
From within a stored procedure:
Call getnum stored procedure
sproc getnum
begin trans
select current seqnum into a variable from Seqtbl where lookupval = 'nosebleed'
update Seqtbl.seqnum++ where lookupval = 'nosebleed'
end trans
I thought of adding a bool column bLock and then having the getnum stored procedure check if the value = false then update the lock (bLock=true) followed by a read, update, and update the lock (bLock = false) without using a transaction. But I am not convinced that ill conceived timing of multiple processes could not interfere with each other.
I do see others using identity columns to achieve similar solutions but it seems that these approaches require one table per LookUp (from the sample schema above) value.
Does anyone suggestions, strategies used to solve similar problems, guidance, or links to send me to school on the important aspects of SQL Server needed to understand a solution to this scenario?
You should get rid of deadlocks if you use just single stament:
declare #id int
update Seqtbl
set #id = seqnum, seqnum = seqnum + 1
where lookupval = 'nosebleed'
The bigger problem here is that you said that there cannot be holes in the sequence. If your actual transaction can be rolled back, then you'll have to include the sequence fetching to the same transaction to be rolled back as well and that's probably going to cause you a lot of blocking, depending on how much many calls there are.
If you're using SQL Server 2012 or newer, you should also look into sequence object, but that's not going to solve the issue with missing values either.
This is a bit long for a comment.
Why are you using a sequence for this? Your analogy to the click-counter "guy" would not suggest a sequence or identity value. Instead, it would suggest inserting the click with an identity column and/or precise creation date. A query can then be used to assign a sequential value when you need it:
select t.*, row_number() over (order by id)
from table t;
You can then use arithmetic to get the value in the range that you want.
I need to replace all occurrences "google.com" that are met in the SQL db table Column1 with "newurl". It can be a full cell value, a part of it (substring of varchar()), can be met even several times in a cell.
Based on SO answer search-and-replace-part-of-string-in-database
this is what I need:
UPDATE
MyTable
SET
Column1 = Replace(Column, 'google.com', 'newurl')
WHERE
xxx
However, in that answer it is mentioned that
You will want to be extremely careful when doing this! I highly recommend doing a backup first.
What are the pitfalls of doing this query? Looks like it does the same what any texteditor would do by clicking on Replace All button. I don't think it is possible in my case to check the errors even with reserve copy as I would like to know possible errors in advance.
Any reasons to be careful with this query?
Again, I expect it replaces all occurences of google.com with 'newurl' in the Column1 of MyTable table in the SQL db.
Thank you.
Just create a test table, as a replica of your original source table, complete the update on there and check results.
You would want to do this as good SQL programming practice to ensure you don't mess up columns that should not be updated.
Another thing you can do is get a count of the records before hand that fit the criteria using a SELECT statement.
Run your update statement and if it's a 1-1 match on count, you should be good to go.
The only thing i can think of that would happen negatively in this respect is that additional columns get updated. Your WHERE clause is not specific for us to see, so there's no way to validate that what you're doing will do what you expect it to.
I think the person posting the answer is just being cautious - This will modify the value in Column1 for every row in MyTable, so make sure you mean it when you execute. Another way to be cautious would be to wrap it in a transaction so you could roll it back if you don't like the results.
I'm working to update a stored procedure that current selects up to n rows, if the rows returned = n, does a select count without the limit, and then returns the original select and the total impacted rows.
Kinda like:
SELECT TOP (#rowsToReturn)
A.data1,
A.data2
FROM
mytable A
SET #maxRows = ##ROWCOUNT
IF #rowsToReturn = ##ROWCOUNT
BEGIN
SET #maxRows = (SELECT COUNT(1) FROM mytableA)
END
I'm wanting reduce this to a single select statement. Based on this question, COUNT(*) OVER() allows this, but it is put on every single row instead of in an output parameter. Maybe something like FOUND_ROWS() in MYSQL, such as a ##TOTALROWCOUNT or such.
As a side note, since the actual select has an order by, the data base will need to already traverse the entire set (to make sure that it gets the correct first n ordered records), so the database should already have this count somewhere.
As #MartinSmith mentioned in a comment on this question, there is no direct (i.e. pure T-SQL) way of getting the total numbers of rows that would be returned while at the same time limiting it. In the past I have done the method of:
dump the query to a temp table to grab ##ROWCOUNT (the total set)
use ROW_NUBMER() AS [ResultID] on the ordered results of the main query
SELECT TOP (n) FROM #Temp ORDER BY [ResultID] or something similar
Of course, the downside here is that you have the disk I/O cost of getting those records into the temp table. Put [tempdb] on SSD? :)
I have also experienced the "run COUNT(*) with the same rest of the query first, then run the regular SELECT" method (as advocated by #Blam), and it is not a "free" re-run of the query:
It is a full re-run in many cases. The issue is that when doing COUNT(*) (hence not returning any fields), the optimizer only needs to worry about indexes in terms of the JOIN, WHERE, GROUP BY, ORDER BY clauses. But when you want some actual data back, that could change the execution plan quite a bit, especially if the indexes used to get the COUNT(*) are not "covering" for the fields in the SELECT list.
The other issue is that even if the indexes are all the same and hence all of the data pages are still in cache, that just saves you from the physical reads. But you still have the logical reads.
I'm not saying this method doesn't work, but I think the method in the Question that only does the COUNT(*) conditionally is far less stressful on the system.
The method advocated by #Gordon is actually functionally very similar to the temp table method I described above: it dumps the full result set to [tempdb] (the INSERTED table is in [tempdb]) to get the full ##ROWCOUNT and then it gets a subset. On the downside, the INSTEAD OF TRIGGER method is:
a lot more work to set up (as in 10x - 20x more): you need a real table to represent each distinct result set, you need a trigger, the trigger needs to either be built dynamically, or get the number of rows to return from some config table, or I suppose it could get it from CONTEXT_INFO() or a temp table. Still, the whole process is quite a few steps and convoluted.
very inefficient: first it does the same amount of work dumping the full result set to a table (i.e. into the INSERTED table--which lives in [tempdb]) but then it does an additional step of selecting the desired subset of records (not really a problem as this should still be in the buffer pool) to go back into the real table. What's worse is that second step is actually double I/O as the operation is also represented in the transaction log for the database where that real table exists. But wait, there's more: what about the next run of the query? You need to clear out this real table. Whether via DELETE or TRUNCATE TABLE, it is another operation that shows up (the amount of representation based on which of those two operations is used) in the transaction log, plus is additional time spent on the additional operation. AND, let's not forget about the step that selects the subset out of INSERTED into the real table: it doesn't have the opportunity to use an index since you can't index the INSERTED and DELETED tables. Not that you always would want to add an index to the temp table, but sometimes it helps (depending on the situation) and you at least have that choice.
overly complicated: what happens when two processes need to run the query at the same time? If they are sharing the same real table to dump into and then select out of for the final output, then there needs to be another column added to distinguish between the SPIDs. It could be ##SPID. Or it could be a GUID created before the initial INSERT into the real table is called (so that it can be passed to the INSTEAD OF trigger via CONTEXT_INFO() or a temp table). Whatever the value is, it would then be used to do the DELETE operation once the final output has been selected. And if not obvious, this part influences a performance issue brought up in the prior bullet: TRUNCATE TABLE cannot be used as it clears the entire table, leaving DELETE FROM dbo.RealTable WHERE ProcessID = #WhateverID; as the only option.
Now, to be fair, it is possible to do the final SELECT from within the trigger itself. This would reduce some of the inefficiency as the data never makes it into the real table and then also never needs to be deleted. It also reduces the over-complication as there should be no need to separate the data by SPID. However, this is a very time-limited solution as the ability to return results from within a trigger is going bye-bye in the next release of SQL Server, so sayeth the MSDN page for the disallow results from triggers Server Configuration Option:
This feature will be removed in the next version of Microsoft SQL Server. Do not use this feature in new development work, and modify applications that currently use this feature as soon as possible. We recommend that you set this value to 1.
The only actual way to do:
the query one time
get a subset of rows
and still get the total row count of the full result set
is to use .Net. If the procs are being called from app code, please see "EDIT 2" at the bottom. If you want to be able to randomly run various stored procedures via ad hoc queries, then it would have to be a SQLCLR stored procedure so that it could be generic and work for any query as stored procedures can return dynamic result sets and functions cannot. The proc would need at least 3 parameters:
#QueryToExec NVARCHAR(MAX)
#RowsToReturn INT
#TotalRows INT OUTPUT
The idea is to use "Context Connection = true;" to make use of the internal / in-process connection. You then do these basic steps:
call ExecuteDataReader()
before you read any rows, do a GetSchemaTable()
from the SchemaTable you get the result set field names and datatypes
from the result set structure you construct a SqlDataRecord
with that SqlDataRecord you call SqlContext.Pipe.SendResultsStart(_DataRecord)
now you start calling Reader.Read()
for each row you call:
Reader.GetValues()
DataRecord.SetValues()
SqlContext.Pipe.SendResultRow(_DataRecord)
RowCounter++
Rather than doing the typical "while (Reader.Read())", you instead include the #RowsToReturn param: while(Reader.Read() && RowCounter < RowsToReturn.Value)
After that while loop, call SqlContext.Pipe.SendResultsEnd() to close the result set (the one that you are sending, not the one you are reading)
then do a second while loop that cycles through the rest of the result, but never gets any of the fields:
while (Reader.Read())
{
RowCounter++;
}
then just set TotalRows = RowCounter; which will pass back the number of rows for the full result set, even though you only returned the top n rows of it :)
Not sure how this performs against the temp table method, the dual call method, or even #M.Ali's method (which I have also tried and kinda like, but the question was specific to not sending the value as a column), but it should be fine and does accomplish the task as requested.
EDIT:
Even better! Another option (a variation on the above C# suggestion) is to use the ##ROWCOUNT from the T-SQL stored procedure, sent as an OUTPUT parameter, rather than cycling through the rest of the rows in the SqlDataReader. So the stored procedure would be similar to:
CREATE PROCEDURE SchemaName.ProcName
(
#Param1 INT,
#Param2 VARCHAR(05),
#RowCount INT OUTPUT = -1 -- default so it doesn't have to be passed in
)
AS
SET NOCOUNT ON;
{any ol' query}
SET #RowCount = ##ROWCOUNT;
Then, in the app code, create a new SqlParameter, Direction = Output, for "#RowCount". The numbered steps above stay the same, except the last two (10 and 11), which change to:
Instead of the 2nd while loop, just call Reader.Close()
Instead of using the RowCounter variable, set TotalRows = (int)RowCountOutputParam.Value;
I have tried this and it does work. But so far I have not had time to test the performance against the other methods.
EDIT 2:
If the T-SQL stored procs are being called from the app layer (i.e. no need for ad hoc execution) then this is actually a much simpler variation of the above C# methods. In this case you don't need to worry about the SqlDataRecord or the SqlContext.Pipe methods. Assuming you already have a SqlDataReader set up to pull back the results, you just need to:
Make sure the T-SQL stored proc has a #RowCount INT OUTPUT = -1 parameter
Make sure to SET #RowCount = ##ROWCOUNT; immediately after the query
Register the OUTPUT param as a SqlParameter having Direction = Output
Use a loop similar to: while(Reader.Read() && RowCounter < RowsToReturn) so that you can stop retrieving results once you have pulled back the desired amount.
Remember to not limit the result in the stored proc (i.e. no TOP (n))
At that point, just like what was mentioned in the first "EDIT" above, just close the SqlDataReader and grab the .Value of the OUTPUT param :).
How about this....
DECLARE #N INT = 10
;WITH CTE AS
(
SELECT
A.data1,
A.data2
FROM mytable A
)
SELECT TOP (#N) * , (SELECT COUNT(*) FROM CTE) Total_Rows
FROM CTE
The last column will be populated with the total number of rows it would have returned without the TOP Clause.
The issue with your requirement is, you are expecting a SINGLE select statement to return a table and also a scalar value. which is not possible.
A Single select statement will return a table or a scalar value. OR you can have two separate selects one returning a Scalar value and other returning a scalar. Choice is yours :)
Just because you think TSQL should have a row count because of a sort doe not mean it does. And if it does it does it is not currently sharing it with the outside world.
What you are missing is this is very efficient
select count(*)
from ...
where ...
select top x
from ...
where ...
order by ...
With the count(*) unless the query is just plain ugly those indexes should be in memory.
It has to perform a count to sort based on what?
Did you actually evaluate any query plans?
If TSQL has to perform a sort then explain the following.
Why is the count(*) 100% of the cost when the second had to do a count anyway?
Just where in that second query plan is there a free opportunity to count?
Why are those query plans so different if they both need to count?
I think there is an arcane way to do what you want. It involves triggers and non-temporary tables. And, I should mention, although I have implemented each piece (for different purposes), I have never put them together for this purpose.
The idea starts with this Stack Overflow question. According to this source, ##ROWCOUNT counts the number of attempted inserts, even when they don't really happen. Now, I must admit that a perusal of available documentation doesn't seem to touch on this topic, so this may or may not be "correct" behavior. This method is relying on this "problem".
So, you could do what you want by:
Creating a new table for the output -- but not a table variable or a temporary table.
Creating an "instead of" trigger that prevents more than #maxRows from going into the table.
Select the query results into the table.
Read ##ROWCOUNT after the select.
Note that you can create the table and trigger using dynamic SQL. You could also create it once, and have the trigger read the #maxRows value from some sort of parameter table. As mentioned before, this needs to be a real table that supports triggers.
I have a script that updates itself every week. I've got a warning from my hosting that I've been overloading the server with the script. The problem, I've gathered is that I use too many UPDATE queries (one for each of my 8000+ users).
It's bad coding, I know. So now I need to lump all the data into one SQL query and update it all at once. I hope that is what will fix my problem.
A quick question. If I add purely add UPDATE queries separated by a semicolon like this:
UPDATE table SET something=3 WHERE id=8; UPDATE table SET something=6 WHERE id=9;
And then update the database with one large SQL code as opposed to querying the database for each update, it will be faster right?
Is this the best way to "bunch" together UPDATE statements? Would this significantly reduce server load?
Make a delimited file with your values and use your equivalent of MySQL's LOAD DATA INFILE. This will be significantly faster than an UPDATE.
LOAD DATA INFILE '/path/to/myfile'
REPLACE INTO TABLE thetable(field1,field2, field3)
//optional field and line delimiters
;
Your best bet is to batch these statements by your "something" field:
UPDATE table SET something=3 WHERE id IN (2,4,6,8)
UPDATE table SET something=4 WHERE id IN (1,3,5,7)
Of course, knowing nothing about your requirements, there is likely a better solution out there...
It will improve IO since there is only one round trip, but the database "effort" will be the same.
A curiosity of SQL is that the following integer expression
(1 -abs(sign(A - B))) = 1 if A == B and 0 otherwise. For convenience lets call this expression _eq(A,B).
So
update table set something = 3*_eq(id,8) + 6* _eq(id,9)
where id in (8,9);
will do what you want with a single update statement.
In the process of fixing a poorly imported database with issues caused by using the wrong database encoding, or something like that.
Anyways, coming back to my question, in order to fix this issues I'm using a query of this form:
UPDATE table_name SET field_name =
replace(field_name,’search_text’,'replace_text’);
And thus, if the table I'm working on has multiple columns I have to call this query for each of the columns. And also, as there is not only one pair of things to run the find and replace on I have to call the query for each of this pairs as well.
So as you can imagine, I end up running tens of queries just to fix one table.
What I was wondering is if there is a way of either combine multiple find and replaces in one query, like, lets say, look for this set of things, and if found, replace with the corresponding pair from this other set of things.
Or if there would be a way to make a query of the form I've shown above, to run somehow recursively, for each column of a table, regardless of their name or number.
Thank you in advance for your support,
titel
Let's try and tackle each of these separately:
If the set of replacements is the same for every column in every table that you need to do this on (or there are only a couple patterns), consider creating a user-defined function that takes a varchar and returns a varchar that just calls replace(replace(#input,'search1','replace1'),'search2','replace2') nested as appropriate.
To update multiple columns at the same time you should be able to do UPDATE table_name SET field_name1 = replace(field_name1,...), field_name2 = replace(field_name2,...) or something similar.
As for running something like that for every column in every table, I'd think it would be easiest to write some code which fetches a list of columns and generates the queries to execute from that.
I don't know of a way to automatically run a search-and-replace on each column, however the problem of multiple pairs of search and replace terms in a single UPDATE query is easily solved by nesting calls to replace():
UPDATE table_name SET field_name =
replace(
replace(
replace(
field_name,
'foo',
'bar'
),
'see',
'what',
),
'I',
'mean?'
)
If you have multiple replaces of different text in the same field, I recommend that you create a table with the current values and what you want them replaced with. (Could be a temp table of some kind if this is a one-time deal; if not, make it a permanent table.) Then join to that table and do the update.
Something like:
update t1
set field1 = t2.newvalue
from table1 t1
join mycrossreferncetable t2 on t1.field1 = t2.oldvalue
Sorry didn't notice this is MySQL, the code is what I would use in SQL Server, my SQL syntax may be different but the technique would be similar.
I wrote a stored procedure that does this. I use this on a per database level, although it would be easy to abstract it to operate globally across a server.
I would just paste this inline, but it would seem that I'm too dense to figure out how to use the markdown deal, so the code is here:
http://www.anovasolutions.com/content/mysql-search-and-replace-stored-procedure