I have a Rails application that holds user data (in an aptly named user_data object). I want to display a summary table that shows me the count of total users and the count of users who are still active (status = 'Active'), created each month for the past 12 months.
In SQL against my Postgres database, I can get the result I want with the following query (the date I use in there is calculated by the application, so you can ignore that aspect):
SELECT total.creation_month,
total.user_count AS total_count,
active.user_count AS active_count
FROM
(SELECT date_trunc('month',"creationDate") AS creation_month,
COUNT("userId") AS user_count
FROM user_data
WHERE "creationDate" >= to_date('2015 12 21', 'YYYY MM DD')
GROUP BY creation_month) AS total
LEFT JOIN
(SELECT date_trunc('month',"creationDate") AS creation_month,
COUNT("userId") AS user_count
FROM user_data
WHERE "creationDate" >= to_date('2015 12 21', 'YYYY MM DD')
AND status = 'Active'
GROUP BY creation_month) AS active
ON total.creation_month = active.creation_month
ORDER BY creation_month ASC
How do I write this query with ActiveRecord?
I previously had just the total user count grouped by month in my display, but I am struggling with how to add in the additional column of active user counts.
My application is on Ruby 2.1.4 and Rails 4.1.6.
I gave up on trying to do this the ActiveRecord way. Instead I just constructed my query into a string and passed the string into
ActiveRecord::Base.connection.execute(sql_string)
This had the side effect that my result set came out as a array instead of a set of objects. So getting at the values went from a syntax (where user_data is the name assigned to a single record from the result set) like
user_data.total_count
to
user_data['total_count']
But that's a minor issue. Not worth the hassle.
SPEC
I need to pick all rooms that don't have a single day with saleable = FALSE in the requested time period(07-09 ~ 07-19):
I have a table room with 1 row per room.
I have a table room_skus with one row per room and day (complete set for the relevant time range).
The column saleable is boolean NOT NULL and date is defined date NOT NULL
SELECT id
FROM room r
WHERE NOT EXISTS (
SELECT 1
FROM room_skus
WHERE date BETWEEN '2016-07-09' AND '2016-07-19'
AND room_id = r.id
AND NOT saleable
GROUP BY 1
);
The above SQL query is working, but I wonder how could I translate it into Rails ORM.
Let's say you have array of room_ids called room_ids:
needed_room_ids = room_ids - RoomSku.where(room_id: room_ids, date: '2016-07-09'..'2016-07-19', sealable: false).pluck(:room_id)
If your model of room_sku is called RoomSku
Updated version:
room_ids = Room.all.select { |record| record.room_skus.present? }.map(&:id)
And then:
needed_room_ids = room_ids - RoomSku.where(room_id: room_ids, date: '2016-07-09'..'2016-07-19', sealable: false).pluck(:room_id)
It won't be one query, but you avoid plain SQL like this.
I don't have any project here to test something like it, but it should work:
Room.where.not(id: RoomSku.where(date: DateTime.parse('2016-07-09').strftime("%Y-%m-%d")..DateTime.parse('2016-07-19').strftime("%Y-%m-%d"), saleable: false).pluck(:room_id))
I hope it helps!
I'm experiencing a race condition in ActiveRecord with PostgreSQL where I'm reading a value then incrementing it and inserting a new record:
num = Foo.where(bar_id: 42).maximum(:number)
Foo.create!({
bar_id: 42,
number: num + 1
})
At scale, multiple threads will simultaneously read then write the same value of number. Wrapping this in a transaction doesn't fix the race condition because the SELECT doesn't lock the table. I can't use an auto increment, because number is not unique, it's only unique given a certain bar_id. I see 3 possible fixes:
Explicitly use a postgres lock (a row-level lock?)
Use a unique constraint and retry on fails (yuck!)
Override save to use a subselect, I.E.
INSERT INTO foo (bar_id, number) VALUES (42, (SELECT MAX(number) + 1 FROM foo WHERE bar_id = 42));
All these solutions seem like I'd be reimplementing large parts of ActiveRecord::Base#save! Is there an easier way?
UPDATE:
I thought I found the answer with Foo.lock(true).where(bar_id: 42).maximum(:number) but that uses SELECT FOR UDPATE which isn't allowed on aggregate queries
UPDATE 2:
I've just been informed by our DBA, that even if we could do INSERT INTO foo (bar_id, number) VALUES (42, (SELECT MAX(number) + 1 FROM foo WHERE bar_id = 42)); that doesn't fix anything, since the SELECT runs in a different lock than the INSERT
Your options are:
Run in SERIALIZABLE isolation. Interdependent transactions will be aborted on commit as having a serialization failure. You'll get lots of error log spam, and you'll be doing lots of retries, but it'll work reliably.
Define a UNIQUE constraint and retry on failure, as you noted. Same issues as above.
If there is a parent object, you can SELECT ... FOR UPDATE the parent object before doing your max query. In this case you'd SELECT 1 FROM bar WHERE bar_id = $1 FOR UPDATE. You are using bar as a lock for all foos with that bar_id. You can then know that it's safe to proceed, so long as every query that's doing your counter increment does this reliably. This can work quite well.
This still does an aggregate query for each call, which (per next option) is unnecessary, but at least it doesn't spam the error log like the above options.
Use a counter table. This is what I'd do. Either in bar, or in a side-table like bar_foo_counter, acquire a row ID using
UPDATE bar_foo_counter SET counter = counter + 1
WHERE bar_id = $1 RETURNING counter
or the less efficient option if your framework can't handle RETURNING:
SELECT counter FROM bar_foo_counter
WHERE bar_id = $1 FOR UPDATE;
UPDATE bar_foo_counter SET counter = $1;
Then, in the same transaction, use the generated counter row for the number. When you commit, the counter table row for that bar_id gets unlocked for the next query to use. If you roll back, the change is discarded.
I recommend the counter approach, using a dedicated side table for the counter instead of adding a column to bar. That's cleaner to model, and means you create less update bloat in bar, which can slow down queries to bar.
I want to write a Stored Proc which will need to perform below steps
Get all the rows from a table where flag= 'Y' and status = != 'PROCESSED'
Update rows from step 1 , set status = 'PROCESSED'
I want to do this because this SP will be called every 5 mins from my java program and i do not want to pick the rows which i have already returned from SP thats why i need to mark them processed.
Something like this?
Retrieve the rows you're interested in. Use the holdlock keyword to ensure nothing can sneak in an extra row between the select and the update. The lock is held until the end of the transaction.
The stored procedure performs the retrieval with the shared lock and then upgrades that to exclusive with the update statement.
When the transaction commits the locks are released.
create proc update_status as
begin transaction
select *
from
t1 holdlock
where
flag = 'Y'
and status != 'PROCESSED'
update t1 set
status = 'PROCESSED'
where
flag = 'Y'
and status != 'PROCESSED'
commit
go
EDIT:
I've narrowed my mysql wait timeout down to this line:
IF #resultsFound > 0 THEN
INSERT INTO product_search_query (QueryText, CategoryId) VALUES (keywords, topLevelCategoryId);
END IF;
Any idea why this would cause a problem? I can't work it out!
I've written a stored proc to search for products in certain categories, due to certain constraints I came across, I was unable to do what I wanted (limiting, but whilst still returning the total number of rows found, with sorting, etc..)
It's meant splits up a string of category Ids, from 1,2,3 in to a temporary table, then builds the full-text search query based on sorting options and limits, executes the query string and then selects out the total number of results.
Now, I know I'm no MySQL guru, very far from it, I've got it working, but I keep getting time outs with product searches etc. So I'm thinking this may be causing some kind of problem?
Does anyone have any ideas how I can tidy this up, or even do it in a much better way that I probably don't know about?
Thanks.
DELIMITER $$
DROP PROCEDURE IF EXISTS `product_search` $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `product_search`(keywords text, categories text, topLevelCategoryId int, sortOrder int, startOffset int, itemsToReturn int)
BEGIN
declare foundPos tinyint unsigned;
declare tmpTxt text;
declare delimLen tinyint unsigned;
declare element text;
declare resultingNum int unsigned;
drop temporary table if exists categoryIds;
create temporary table categoryIds
(
`CategoryId` int
) engine = memory;
set tmpTxt = categories;
set foundPos = instr(tmpTxt, ',');
while foundPos <> 0 do
set element = substring(tmpTxt, 1, foundPos-1);
set tmpTxt = substring(tmpTxt, foundPos+1);
set resultingNum = cast(trim(element) as unsigned);
insert into categoryIds (`CategoryId`) values (resultingNum);
set foundPos = instr(tmpTxt,',');
end while;
if tmpTxt <> '' then
insert into categoryIds (`CategoryId`) values (tmpTxt);
end if;
CASE
WHEN sortOrder = 0 THEN
SET #sortString = "ProductResult_Relevance DESC";
WHEN sortOrder = 1 THEN
SET #sortString = "ProductResult_Price ASC";
WHEN sortOrder = 2 THEN
SET #sortString = "ProductResult_Price DESC";
WHEN sortOrder = 3 THEN
SET #sortString = "ProductResult_StockStatus ASC";
END CASE;
SET #theSelect = CONCAT(CONCAT("
SELECT SQL_CALC_FOUND_ROWS
supplier.SupplierId as Supplier_SupplierId,
supplier.Name as Supplier_Name,
supplier.ImageName as Supplier_ImageName,
product_result.ProductId as ProductResult_ProductId,
product_result.SupplierId as ProductResult_SupplierId,
product_result.Name as ProductResult_Name,
product_result.Description as ProductResult_Description,
product_result.ThumbnailUrl as ProductResult_ThumbnailUrl,
product_result.Price as ProductResult_Price,
product_result.DeliveryPrice as ProductResult_DeliveryPrice,
product_result.StockStatus as ProductResult_StockStatus,
product_result.TrackUrl as ProductResult_TrackUrl,
product_result.LastUpdated as ProductResult_LastUpdated,
MATCH(product_result.Name) AGAINST(?) AS ProductResult_Relevance
FROM
product_latest_state product_result
JOIN
supplier ON product_result.SupplierId = supplier.SupplierId
JOIN
category_product ON product_result.ProductId = category_product.ProductId
WHERE
MATCH(product_result.Name) AGAINST (?)
AND
category_product.CategoryId IN (select CategoryId from categoryIds)
ORDER BY
", #sortString), "
LIMIT ?, ?;
");
set #keywords = keywords;
set #startOffset = startOffset;
set #itemsToReturn = itemsToReturn;
PREPARE TheSelect FROM #theSelect;
EXECUTE TheSelect USING #keywords, #keywords, #startOffset, #itemsToReturn;
SET #resultsFound = FOUND_ROWS();
SELECT #resultsFound as 'TotalResults';
IF #resultsFound > 0 THEN
INSERT INTO product_search_query (QueryText, CategoryId) VALUES (keywords, topLevelCategoryId);
END IF;
END $$
DELIMITER ;
Any help is very very much appreciated!
There is little you can do with this query.
Try this:
Create a PRIMARY KEY on categoryIds (categoryId)
Make sure that supplier (supplied_id) is a PRIMARY KEY
Make sure that category_product (ProductID, CategoryID) (in this order) is a PRIMARY KEY, or you have an index with ProductID leading.
Update:
If it's INSERT that causes the problem and product_search_query in a MyISAM table the issue can be with MyISAM locking.
MyISAM locks the whole table if it decides to insert a row into a free block in the middle of the table which can cause the timeouts.
Try using INSERT DELAYED instead:
IF #resultsFound > 0 THEN
INSERT DELAYED INTO product_search_query (QueryText, CategoryId) VALUES (keywords, topLevelCategoryId);
END IF;
This will put the records into the insertion queue and return immediately. The record will be added later asynchronously.
Note that you may lose information if the server dies after the command is issued but before the records are actually inserted.
Update:
Since your table is InnoDB, it may be an issue with table locking. INSERT DELAYED is not supported on InnoDB.
Depending on the nature of the query, DML queries on InnoDB table may place gap locks which will lock the inserts.
For instance:
CREATE TABLE t_lock (id INT NOT NULL PRIMARY KEY, val INT NOT NULL) ENGINE=InnoDB;
INSERT
INTO t_lock
VALUES
(1, 1),
(2, 2);
This query performs ref scans and places the locks on individual records:
-- Session 1
START TRANSACTION;
UPDATE t_lock
SET val = 3
WHERE id IN (1, 2)
-- Session 2
START TRANSACTION;
INSERT
INTO t_lock
VALUES (3, 3)
-- Success
This query, while doing the same, performs a range scan and places a gap lock after key value 2, which will not let insert key value 3:
-- Session 1
START TRANSACTION;
UPDATE t_lock
SET val = 3
WHERE id BETWEEN 1 AND 2
-- Session 2
START TRANSACTION;
INSERT
INTO t_lock
VALUES (3, 3)
-- Locks
Try wrapping your EXECUTE with the following:
SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
EXECUTE TheSelect USING #keywords, #keywords, #startOffset, #itemsToReturn;
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ ;
I do something similiar in TSQL for all report stored proc and searches where repeatable reads aren't important to reduce locking/blocking issues with other processes running on the database.
Turn on slow queries, that will give you an idea of what is taking so long to execute that there is a timeout.
http://dev.mysql.com/doc/refman/5.1/en/slow-query-log.html
Pick the slowest query and optimise that. then run for a while and repeat.
There is some excellent information and tools here http://hackmysql.com/nontech
DC
UPDATE:
Either you have a network problem causing the timeout, if you are using a local mysql instance then that is unlikely, OR something is locking a table for far too long causing a timeout. the process that is locking the table or tables for far too long will be listed in the slow log as a slow query. you can also get the slow log query to display any queries that fail to use an index resulting in an inefficient query.
If you can get the problem to occur while you are present then you can also use a tool like phpmyadmin or the commandline to run "SHOW PROCESSLIST\G" this will give you a list of what queries are running while the problem is occurring.
You think the problem is in your insert statement, therefore something is locking that table. therefore you need to find what is locking that table, therefore you need to find what is running so slow its locking the table for far too long. Slow queries is one way to do that.
Other things to look at
CPU - is it idle or running at full pelt
IO - is io causing holdups
RAM - are you swapping all the time (will cause excessive io)
Does the table product_search_query use an index?
What is the primary key?
If your index uses strings that are too long? you may build a huge index file that causes very slow inserts (slow query log will also show that)
And yes the problem may be elsewhere, but you must start somewhere mustn't you.
DC