I have been searching for an answer to this today, and it seems the best approach divides opinion somewhat.
I have 150,000 records that I need to retrieve from an Oracle database using JDBC. Is it better to retrieve the data using one select query and allowing the JDBC driver to take care of transferring the records from the database using Oracle cursor and default fetchSize - OR to split up the query into batches using LIMIT / OFFSET?
With the LIMIT / OFFSET option, I think the pros are that you can take control over the number of results you return in each chunk. The cons are that the query is executed multiple times, and you also need to run a COUNT(*) up front using the same query to calculate the number of iterations required.
The pros of retrieving all at once are that you rely on the JDBC driver to manage the retrieval of data from the database. The cons are that the setFetchSize() hint can sometimes be ignored meaning that we could end up with a huge resultSet containing all 150,000 records at once!!
Would be great to hear some real life experiences solving similar issues, and recommendations would be much appreciated.
The native way in Oracle JDBC is to use the prepareStatement for the query, executeQuery and fetch
in a loop the results with defined fetchSize
Yes, of course the details are Oracle Database and JDBC Driver Version dependent and in some case the required fetchSize
can be ignored. But the typical problem is that the required fetch size is reset to fetchSize = 1 and you effectively makes a round trip for each record. (not that you get all records at once).
Your alternative with LIMIT seems to be meaningfull on the first view. But if you investigate the implementation you will probably decide to not use it.
Say you will divide the result set in 15 chunks 10K each:
You open 15 queries, each of them on average with a half of the resource consumption as the original query (OFFSET select the data and skips them).
So the only think you will reach is that the processing will take aproximatly 7,5x more time.
Best Practice
Take your query, write a simple script with JDBC fetch, use 10046 trace to see the effective used fetch size.
Test with a range of fetch sizes and observe the perfomance; choose the optimal one.
my preference is to maintain a safe execution time with the ability to continue if interrupted. i prefer this approach because it is future proof and respects memory and execution time limits. remember you're not planning for today, you're planning for 6m down the road. what may be 150,000 today may be 1.5m in 6 months.
i use a length + 1 recipe to know if there is more to fetch, although the count query will enable you to do a progress bar in % if that is important.
when considering 150,000 record result set, this is a memory pressure question. this will depend on the average size of each row. if it is a row with three integers, that's small. if it is a row with a bunch of text elements to store user profile details then that's potentially very large. so be prudent with what fields you're pulling.
also need to ask - you may not need to pull all the records all the time. it may be useful to apply a sync pattern. to only pull records with an updated date newer than your last pull.
Related
I'm trying to develop a Scala microservice for data management for an Oracle database. I'm using JDBC drivers to connect to it.
Reading the answers to the performance questions regarding JDBC driver compared to the .NET one, I've understood that one of the more effective vehicle to tune the JDBC reading performance is to set the Fetch Size through the method ResultSet.setFetchSize.
I've tried connecting to an Oracle database to fetch real data for a real business case, with a fixed number of record returned by the DB, and I've measured an exponential behavior of the elapsed time. In particular, fetching 10,000 rows from the database without setting the fetch size resulting in a ridicolously large amount of fetch time, but specifying a fetch size larger than 1,000 resulting in a little amount of time gained (roughly 100 ms over 1 s).
Here's my questions regarding this topic:
I suppose that incrementing too much the fetch size would consume resources inopportunely for a little gain, so is there an even rough method to estimate the size of the ResultSet before actually fetching it? I've read about the following technique:
result.last();
result.getRow();
but this would mean scroll the entire ResultSet, and I was wondering if there's any even rough accurate technique to evaluate the count;
I've estimated that a good fetch size would be 1/10th of the number of record selected, but is there a documented rule to try to automatically estimate the correct fetch size for the largest number of cases?
Please do not set fetch size too large, unless you have network bottleneck between application and database. The larger the fetch size, the more memory consumed.
In my experience, 1024 - 2048 will lead to best performance most of the time. See
https://docs.oracle.com/javase/tutorial/jdbc/basics/retrieving.html discussing some details, but the default setting is usually best.
Do not try to get the total numbers of rows in the result set, it is not the best practice.
And finally, I want to point out that based on the hundreds of thousands of time optimize about JVM and jit, the bottleneck seems never happens on fetch size of JDBC after you set it with 1000-2000, but on the SQL performance, applications or resource limit and etc.
I am using mybatis 3.1.0 jar along with spring mybatis jar.
It is taking me 16 seconds to pullout the 44000 rows into 3000 entity objects.
Time Taken: Normal query execution time : 11 seconds
Any suggestions to increase the performance?
Thanks.
First off, do some analysis to see how much of your time is spent (a) pulling out data, e.g. run the query in your standalone DB tool and benchmark that, and then (b) how much is spent marshalling the data into your objects.
Subsequent steps will depend on which of (a) or (b) appears least performant.
If (a), then spend time tuning the query - indexes on tables, denormalise underlying structure.
If (b) consider a flatter or less heavily populated model.
Edit:
extra thoughts on DB side to reduce 11s duration:
check your query is hitting indexes at every point, not doing any table scans
check your query pulls back the minimum fields, e.g. if you just need 8 fields, don't pull back 20
check your query isn't doing inefficient subqueries if you're on MySql (are you?)
check your query isn't calling any unnecessary functions or subroutines (probably unlikely but worth mentioning...)
extra thoughts on object side:
avoid setting object fields you don't need
extra general thoughts:
add some logging for, e.g. a single case (or for, say, 100) to see where time's spent in the application, you need general code optimisation techniques here - anything that's a loop, or isn't 100% necessary, look at changing/removing
if performance is most important consideration, consider changing your requirement - structure your page, or screen, differently, for example to allow you to make a faster DB retrieval realistic
2nd edit - do you really need to use all 44,000 rows and the 3,000 objects they feed? Could you get by with fewer, e.g. break them into 10 groups and paginate with them? (wild guess, your app might do nothing of that sort)
Certain measures on mybatis configuration to handle situations like these,
1)Set fetchSize to a considerable amount.I set my fetchsize as 1000
2)Not to retrieve and map all columns(mapping only necessary columns reduced the time)
3)Make use of nested joins instead of nested selects.
I have a very large set of data (~3 million records) which needs to be merged with updates and new records on a daily schedule. I have a stored procedure that actually breaks up the record set into 1000 record chunks and uses the MERGE command with temp tables in an attempt to avoid locking the live table while the data is updating. The problem is that it doesn't exactly help. The table still "locks up" and our website that uses the data receives timeouts when attempting to access the data. I even tried splitting it up into 100 record chunks and even tried a WAITFOR DELAY '000:00:5' to see if it would help to pause between merging the chunks. It's still rather sluggish.
I'm looking for any suggestions, best practices, or examples on how to merge large sets of data without locking the tables.
Thanks
Change your front end to use NOLOCK or READ UNCOMMITTED when doing the selects.
You can't NOLOCK MERGE,INSERT, or UPDATE as the records must be locked in order to perform the update. However, you can NOLOCK the SELECTS.
Note that you should use this with caution. If dirty reads are okay, then go ahead. However, if the reads require the updated data then you need to go down a different path and figure out exactly why merging 3M records is causing an issue.
I'd be willing to bet that most of the time is spent reading data from the disk during the merge command and/or working around low memory situations. You might be better off simply stuffing more ram into your database server.
An ideal amount would be to have enough ram to pull the whole database into memory as needed. For example, if you have a 4GB database, then make sure you have 8GB of RAM.. in an x64 server of course.
I'm afraid that I've quite the opposite experience. We were performing updates and insertions where the source table had only a fraction of the number of rows as the target table, which was in the millions.
When we combined the source table records across the entire operational window and then performed the MERGE just once, we saw a 500% increase in performance. My explanation for this is that you are paying for the up front analysis of the MERGE command just once instead of over and over again in a tight loop.
Furthermore, I am certain that merging 1.6 million rows (source) into 7 million rows (target), as opposed to 400 rows into 7 million rows over 4000 distinct operations (in our case) leverages the capabilities of the SQL server engine much better. Again, a fair amount of the work is in the analysis of the two data sets and this is done only once.
Another question I have to ask is well is whether you are aware that the MERGE command performs much better with indexes on both the source and target tables? I would like to refer you to the following link:
http://msdn.microsoft.com/en-us/library/cc879317(v=SQL.100).aspx
From personal experience, the main problem with MERGE is that since it does page lock it precludes any concurrency in your INSERTs directed to a table. So if you go down this road it is fundamental that you batch all updates that will hit a table in a single writer.
For example: we had a table on which INSERT took a crazy 0.2 seconds per entry, most of this time seemingly being wasted on transaction latching, so we switched this over to using MERGE and some quick tests showed that it allowed us to insert 256 entries in 0.4 seconds or even 512 in 0.5 seconds, we tested this with load generators and all seemed to be fine, until it hit production and everything blocked to hell on the page locks, resulting in a much lower total throughput than with the individual INSERTs.
The solution was to not only batch the entries from a single producer in a MERGE operation, but also to batch the batch from producers going to individual DB in a single MERGE operation through an additional level of queue (previously also a single connection per DB, but using MARS to interleave all the producers call to the stored procedure doing the actual MERGE transaction), this way we were then able to handle many thousands of INSERTs per second without problem.
Having the NOLOCK hints on all of your front-end reads is an absolute must, always.
I am working on a application. It is in its initial stage so the number of records in table is not large, but later on it will have around 1 million records in the same table.
I want to know what points I should consider while writing select query which will fetch a huge amount of data from table so it does not slow down performance.
First rule:
Don't fetch huge amounts of data back to the application.
Unless you are going to display every single one of the items in the huge amount of data, do not fetch it. Communication between the DBMS and the application is (relatively) slow, so avoid it when possible. It isn't so slow that you shouldn't use the DBMS or anything like that, but if you can reduce the amount of data flowing between DBMS and application, the overall performance will usually improve.
Often, one easy way to do this is to list only those columns you actually need in the application, rather than using 'SELECT *' to retrieve all columns when you'll only use 4 of the 24 that exist.
Second rule:
Try to ensure that the DBMS does not have to look at huge amounts of data.
To the extent possible, minimize the work that the DBMS has to do. It is busy, and typically it is busy on behalf of many people at any given time. If you can reduce the amount of work that the DBMS has to do to process your query, everyone will be happier.
Consider things like ensuring you have appropriate indexes on the table - not too few, not too many. Designed judiciously, indexes can greatly improve the performance of many queries. Always remember, though, that each index has to be maintained, so inserts, deletes and updates are slower when there are more indexes to manage on a given table.
(I should mention: none of this advice is specific to Oracle - you can apply it to any DBMS.)
To get good performance with a database there is a lot of things you need to have in mind. At first, it is the design, and here you should primary think about normalization and denormalization (split up tables but still not as much as performance heavy joins are required).
There are often a big bunch of tuning when it comes to performance. However, 80% of the performance is determined from the SQL-code. Below are some links that might help you.
http://www.smart-soft.co.uk/Oracle/oracle-performance-tuning-part7.htm
http://www.orafaq.com/wiki/Oracle_database_Performance_Tuning_FAQ
A few points to remember:
Fetch only the columns you need to use on the client side.
Ensure you set up the correct indexes that are going to help you find records. These can be done later, but it is better to plan for them if you can.
Ensure you have properly accounted for column widths and data sizes. Don't use an INT when a TINYINT will hold all possible values. A row with 100 TINYINT fields will fetch faster than a row with 100 INT fields, and you'll also be able to fetch more rows per read.
Depending on how clean you need the data to be, it may be permissable to do a "dirty read", where the database fetches data while an update is in progress. This can speed things up significantly in some cases, though it means the data you get might not be the absolute latest.
Give your DBA beer. And hugs.
Jason
We have about 10K rows in a table. We want to have a form where we have a select drop down that contains distinct values of a given column in this table. We have an index on the column in question.
To increase performance I created a little cache table that contains the distinct values so we didn't need to do a select distinct field from table against 10K rows. Surprisingly it seems doing select * from cachetable (10 rows) is no faster than doing the select distinct against 10K rows. Why is this? Is the index doing all the work? At what number of rows in our main table will there be a performance improvement by querying the cache table?
For a DB, 10K rows is nothing. You're not seeing much difference because the actual calculation time is minimal, with most of it consumed by other, constant, overhead.
It's difficult to predict when you'd start noticing a difference, but it would probably be at around a million rows.
If you've already set up caching and it's not detrimental, you may as well leave it in.
10k rows is not much... start caring when you reach 500k ~ 1 million rows.
Indexes do a great job, specially if you just have 10 different values for that index.
This depends on numerous factors - the amount of memory your DB has, the size of the rows in the table, use of a parameterised query and so forth, but generally 10K is not a lot of rows and particularly if the table is well indexed then it's not going to cause any modern RDBMS any sweat at all.
As a rule of thumb I would generally only start paying close attention to performance issues on a table when it passes the 100K rows mark, and 500K doesn't usually cause much of a problem if indexed correctly and accessed by such. Performance usually tends to fall off catastrophically on large tables - you may be fine on 500K rows but crawling on 600K - but you have a long way to go before you are at all likely to hit such problems.
Is the index doing all the work?
You can tell how the query is being executed by viewing the execution plan.
For example, try this:
explain plan for select distinct field from table;
select * from table(dbms_xplan.display);
I notice that you didn't include an ORDER BY on that. If you do not include ORDER BY then the order of the result set may be random, particularly if oracle uses the HASH algorithm for making a distinct list. You ought to check that.
So I'd look at the execution plans for the original query that you think is using an index, and at the one based on the cache table. Maybe post them and we can comment on what's really going on.
Incidentaly, the cache table would usually be implemented as a materialised view, particularly if the master table is generally pretty static.
Serious premature optimization. Just let the database do its job, maybe with some tweaking to the configuration (especially if it's MySQL, which has several cache types and settings).
Your query in 10K rows most probably uses HASH SORT UNIQUE.
As 10K most probably fit into db_buffers and hash_area_size, all operations are performed in memory, and you won't note any difference.
But if the query will be used as a part of a more complex query, or will be swapped out by other data, you may need disk I/O to access the data, which will slow your query down.
Run your query in a loop in several sessions (as many sessions as there will be users connected), and see how it performs in that case.
For future plans and for scalability, you may want to look into an indexing service that uses pure memory or something faster than the TCP DB round-trip. A lot of people (including myself) use Lucene to achieve this by normalizing the data into flat files.
Lucene has a built-in Ram Drive directory indexer, which can build the index all in memory - removing the dependency on the file system, and greatly increasing speed.
Lately, I've architected systems that have a single Ram drive index wrapped by a Webservice. Then, I have my Ajax-like dropdowns query into that Webservice for high availability and high speed - no db layer, no file system, just pure memory and if remote tcp packet speed.
If you have an index on the column, then all the values are in the index and the dbms never has to look in the table. It just looks in the index which just has 10 entries. If this is mostly read only data, then cache it in memory. Caching helps scalability and a lot by relieving the database of work. A query that is quick on a database with no users, might perform poorly if a 30 queries are going on at the same time.