What makes Oracle more scalable? - oracle

Oracle seems to have a reputation for being more scalable than other RDBMSes. After working with it a bit, I can say that it's more complex than other RDBMSes, but I haven't really seen anything that makes it more scalable than other RDBMSes. But then again, I haven't really worked on it in a whole lot of depth.
What features does Oracle have that are more scalable?

Oracle's RAC architecture is what makes it scalable where it can load balance across nodes and parallel queries can be split up and pushed to other nodes for processing.
Some of the tricks like loading blocks from another node's buffer cache instead of going to disc make performance a lot more scalable.
Also, the maintainability of RAC with rolling upgrades help make the operation of a large system more sane.
There is also a different aspect of scalability - storage scalability. ASM makes increasing the storage capacity very straightforward. A well designed ASM based solution, should scale past the 100s of terabyte size without needing to do anything very special.
Whether these make Oracle more scalable than other RDBMSs, I don't know. But I think I would feel less happy about trying to scale up a non-Oracle database.

Cursor sharing is (or was) a big advantage over the competition.
Basically, the same query plan is used for matching queries. An application will have a standard set of queries it issue (eg get the orders for this customer id). The simple way is to treat every query individually, so if you see 'SELECT * FROM ORDERS WHERE CUSTOMER_ID = :b1', you look at whether table ORDERS has an index on CUSTOMER_ID etc. As a result, you can spend as much work looking up meta data to get a query plan as actually retrieving the data. With simple keyed lookups, a query plan is easy. Complex queries with multiple tables joined on skewed columns are harder.
Oracle has a cache of query plans, and older/less used plans are aged out as new ones are required.
If you don't cache query plans, there's a limit to how smart you can make your optimizer as the more smarts you code into it, the bigger impact you have on each query processed. Caching queries means you only incur that overhead the first time you see the query.
The 'downside' is that for cursor sharing to be effective you need to use bind variables. Some programmers don't realise that and write code that doesn't get shared and then complain that Oracle isn't as fast as mySQL.
Another advantage of Oracle is the UNDO log. As a change is done, the 'old version' of the data is written to an undo log. Other database keep old versions of the record in the same place as the record. This requires VACUUM style cleanup operations or you bump into space and organisation issues. This is most relevant in databases with high update or delete activity.
Also Oracle doesn't have a central lock registry. A lock bit is stored on each individual data record. SELECT doesn't take a lock. In databases where SELECT locks, you could have multiple users reading data and locking each other or preventing updates, introducing scalability limits. Other databases would lock a record when a SELECT was done to ensure that no-one else could change that data item (so it would be consistent if the same query or transaction looked at the table again). Oracle uses UNDO for its read consistency model (ie looking up the data as it appeared at a specific point in time).

Tom Kyte's "Expert Oracle Database Architecture" from Apress does a good job of describing Oracle's architecture, with some comparisons with other rDBMSs. Worth reading.

Related

Use Vertica Database for OLTP data?

Can Vertica Database be used for OLTP data?
And if so what are the pros and cons on doing this?
Looking for a Vertica vs Oracle fight :)Since Oracle license is so costly, would Vertica do it job for a better price ?
thx all
Using Vertica as a transactional database is a bad idea. It's designed to be a data warehousing tool. Essentially, it reads and writes data in an optimized fashion. Lots of transactions? That's not what it is designed to do.
I would recommend that you look into VoltDB. Michael Stonebreaker who is the force behind Vertica founded that company as well. His basic philosophy is that Oracle, SQL Server, et al do not do well for high performance since they are designed to do everything. The future is having databases designed for specific tasks.
So he had some concepts for a data warehousing which became Vertica. For transactional databases, there's VoltDB. Not owned by HP, for the record.
For the record, I haven't used VoltDB. From what I know, it isn't as mature as Vertica is as a solution but it looks like it has a ton of promise.
HP Vertica is a column store database. The nature of the way that data is organised within a column store does not lend itself to rapid writes.
HP Vertica gets around this by having a WOS (Write Optimised Store) and ROS (Read Optimised Store which is file based).
Data is moved out of the WOS into the ROS fairly rapidly and the ROS itself has a "merge up" process that takes small ROS files and merges them together to form larger and therefore more easily scanned files.
If you tried to use Vertica for OLTP then what would happen would be that you'd get loads of ROS containers and possibly hit the default limit of 1024 ROS containers very quickly.
If you fronted the store with some form a queuing mechanism to pass through records in larger batches then this would result in fewer and larger ROS files. It would work but if you wanted to take your OLTP system to be reading very close to its writing activity it would not fit the use case.
The WOS/ROS mechanism is a neat work around for the fundamental performance penalty of writes in a column store DB but fundamentally Vertica is not an OLTP DB but rather a data mart technology that can ingest data in near real time
I think there are different ways to read into this question.
Can you use Vertica as an OLTP database?
First I'll define this question a bit. An OLTP database means the database itself is responsible for the transaction processing, not simply receiving somewhat normalized data.
My answer here is absolutely not, unless perhaps it is a single user database. There is practically no RI, no RI locking, table locks on DELETE/UPDATE, and you're likely to accumulate a delete vector in normal OLTP type usage.
You can work around some of these with some extensive middleware programming (distributed locks, heavy avoidance of DELETE/UPDATE, etc). But why? There are tons of options out there that are not Oracle, don't carry a huge price tag but give you everything you need for OLTP.
Can you use Vertica to ingest and query OLTP data?
Yes, definitely. Best to use Vertica towards its strengths, though. Queries in Vertica tend to have a fair amount of overhead, and you can plow through large amounts of data with ease, even normalized. I would not be using Vertica to primary run point queries, grabbing a few rows here and there. It isn't that you can't, but you can't with the same concurrency as other databases that are meant for this purpose.
TL;DR Use the right tool for the right job. I really love using Vertica, but just because I like to swing a hammer doesn't mean that every problem is a nail.
This question is a little old now but i'll share my experience.
I would not suggest vertica as OLTP unless you very carefully consider your workload.
As mentioned in other answers, Vertica has 2 types of storage. ROS is the Read Optimized Storage and WOS is the Write Optimized Storage. WOS is purely in memory so it performs better for inserts but queries slower as all the small updates need to be queried and unioned. Vertica can handle small loads in theory but in practice it didn't work out very well for us performance wise. Also there are drawbacks to WOS namely being that when the database fails WOS is not necessarily preserved when it rolls back to last good epoch. (ROS isn't either but in practice you lose a lot less from ROS).
ROS is a lot more reliable and gives better read performance but you will never be able to handle more than a certain number of queries without a careful design. Although vertica is horizontally scalable, in practice large tables get segmented across all nodes and therefore queries must run on all nodes. So adding more nodes doesn't mean handling more concurrent queries it just means less work per query. If your tables are small enough to be unsegmented then this might not be an issue for you.
Also worth noting is the OLTP typically implies lots concurrent transactions so you'll need to plan resource pools very carefully. By default vertica has a planned concurrency for the general resource pool of the minimum of number of cores per server or RAM/2GB. Essentially what this value does is determine the default memory allocation PER NODE for a segmented query. Therefore by default vertica will not let you run more queries than cores. You can adjust this value but once you hit a cap on memory theres no much you can do because the memory is allocated per node so adding more nodes doesn't even help. If you hit any errors at all for resource pool memory allocations that is the first config your should look at.
Additionally, Vertica is bad with deletes and updates (which resolve to a delete and an insert in the background) so if these are a regular part of your workload then Vertica is probably a bad choice. Personally we use MySQL for our dimension tables that require deletes/updates and then sync that data periodically into vertica to use for joins.
Personally I use Vertica as an OLTP-ish realtime-ish database. We batch our loads into 5 minute intervals which makes vertica happy in terms of how many/large the inserts are. These batches are inserted using COPY DIRECT so that they avoid WOS entirely (only do this if they are large batches as this forces ROS container creation and can be bad if you do it too often). As many projections as we can have are unsegmented to allow better scale out since this makes queries hit only 1 node and allocate memory on only 1 node. It has worked well for us so far and we load about 5 billion rows a day with realtime querying from our UI.
Up_one - considering the telecom use-case - are you doing CDR or something else?
To answer your original question yes Vertica may be a great fit but it depends on how you are loading the data, how you are doing updates, what your data size is and what your SLA is. I am really familiar in this space because I implemented Vertica at a telecom that I worked for at the time.

Performance Implications of Using Oracle DBMS_WM.EnableVersioning

The command to enable versioning (part of what Oracle calls Workspace Management) in Oracle (DBMS_WM.EnableVersioning) creates non-materialized views, which cannot be indexed. Will this kill performance, or will the indexes for the _AUX, _LT, and _LCK tables be used when the views are queried?
Are there significant performance issues in addition to indexing when enabling versioning?
I am using Oracle 11g.
As with most things, it depends.
Do you have queries today that need to do table scans to fetch all their data? Or is everything going to go through an index?
What is the use case? Are you using Workspace Manager to support long-running transactions? Or to maintain history data in a single LIVE workspace?
How frequently do rows change? How many versions of a row are you planning to keep?
If you have existing queries that will do table scans, the table is rebuilt every night, and you plan on keeping history data forever, you're likely going to have major performance issues. If all your queries use indexes to access data, rows change infrequently, and you just intend to retain a few versions of history, the indexes on the underlying tables should be sufficient.
We've used Workspace Manager to maintain history on relatively slowly changing tables forever as well as relatively fast changing tables for a month. And we've used it to maintain discrete savepoints across tables in a few applications so that users can permanently save the state of application data at interesting points in time. In general, we've been satisfied with performance though complex queries will occasionally go off into the weeds when the optimizer gets confused.
Since you're on 11g, you may also consider Total Recall. It's an extra-cost option on top of the enterprise license but it provides a much more efficient architecture for tracking changes to data over time assuming that you intend to store all changes for a fixed period of time. On the other hand, you're more limited in the DDL you can issue without causing history to be discarded which tends to be a rather serious constrain in the applications I've worked on.

Oracle select query performance

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

Does Oracle 11g automatically index fields frequently used for full table scans?

I have an app using an Oracle 11g database. I have a fairly large table (~50k rows) which I query thus:
SELECT omg, ponies FROM table WHERE x = 4
Field x was not indexed, I discovered. This query happens a lot, but the thing is that the performance wasn't too bad. Adding an index on x did make the queries approximately twice as fast, which is far less than I expected. On, say, MySQL, it would've made the query ten times faster, at the very least. (Edit: I did test this on MySQL, and there saw a huge difference.)
I'm suspecting Oracle adds some kind of automatic index when it detects that I query a non-indexed field often. Am I correct? I can find nothing even implying this in the docs.
As has already been indicated, Oracle11g does NOT dynamically build indexes based on prior experience. It is certainly possible and indeed happens often that adding an index under the right conditions will produce the order of magnitude improvement you note.
But as has also already been noted, 50K (seemingly short?) rows is nothing to Oracle. The Oracle database in fact has a great deal of intelligence that allows it to scan data without indexes most efficiently. Every new release of the Oracle RDBMS gets better at moving large amounts of data. I would suggest to you that the reason Oracle was so close to its "best" timing even without the index as compared to MySQL is that Oracle is just a more intelligent database under the covers.
However, the Oracle RDBMS does have many features that touch upon the subject area you have opened. For example:
10g introduced a feature called AUTOMATIC SQL TUNING which is exposed via a gui known as the SQL TUNING ADVISOR. This feature is intended to analyze queries on its own, in depth and includes the ability to do WHAT-IF analysis of alternative query plans. This includes simulation of indexes which do not actually exist. However, this would not explain any performance differences you have seen because the feature needs to be turned on and it does not actually build any indexes, it only makes recommendations for the DBA to make indexes, among other things.
11g includes AUTOMATIC STATISTICS GATHERING which when enabled will automatically collect statistics on database objects as it deems necessary based on activity on those objects.
Thus the Oracle RDBMS is doing what you have suggested, dynamically altering its environment on its own based on its experience with your workload over time in order to improve performance. Creating indexes on the fly is just not one of the things is does yet. As an aside, this has been hinted to by Oracle in private sevearl times so I figure it is in the works for some future release.
Does Oracle 11g automatically index fields frequently used for full table scans?
No.
In regards the MySQL issue, what storage engine you use can make a difference.
"MyISAM relies on the operating system for caching reads and writes to the data rows while InnoDB does this within the engine itself"
Oracle will cache the table/data rows, so it won't need to hit the disk. depending on the OS and hardware, there's a chance that MySQL MyISAM had to physically read the data off the disk each time.
~50K rows, depending greatly on how big each row is, could conceivably be stored in under 1000 blocks, which could be quickly read into the buffer cache by a full table scan (FTS) in under 50 multi-block reads.
Adding appropriate index(es) will allow queries on the table to scale smoothly as the data volume and/or access frequency goes up.
"Adding an index on x did make the
queries approximately twice as fast,
which is far less than I expected. On,
say, MySQL, it would've made the query
ten times faster, at the very least."
How many distinct values of X are there? Are they clustered in one part of the table or spread evenly throughout it?
Indexes are not some voodoo device: they must obey the laws of physics.
edit
"Duplicates could appear, but as it
is, there are none."
If that column has neither a unique constraint nor a unique index the optimizer will choose an execution path on the basis that there could be duplicate values in that column. This is the value of declaring the data model as accuratley as possible: the provision of metadata to the optimizer. Keeping the statistics up to date is also very useful in this regard.
You should have a look at the estimated execution plan for your query, before and after the index has been created. (Also, make sure that the statistics are up-to-date on your table.) That will tell you what exactly is happening and why performance is what it is.
50k rows is not that big of a table, so I wouldn't be surprised if the performance was decent even without the index. Thus adding the index to equation can't really bring much improvement to query execution speed.

How to minimize transaction overhead in Oracle?

I have to simultaneously load data into a table and run queries on it. Because of data nature, I can trade integrity for performance. How can I minimize the overhead of transactions?
Unfortunately, alternatives like MySQL cannot be used (due to non-technical reasons).
Other than the general optimization practices that apply to all databases such as eliminating full table scans, removing unused or inefficient indexes, etc., etc., here are a few things you can do.
Run in No Archive Log mode. This sacrifices recoverability for speed.
For inserts use the /*+ APPEND */ hint. This puts data into the table above the high water mark which does not create UNDO. The disadvantage is that existing free space is not used.
On the hardware side, RAID 0 over a larger number of smaller disks will give you the best insert performance, but depending on your usage RAID 10 with its better read performance may provide a better fit.
This said, I don't think you will gain much from any of these changes.
Perhaps I'm missing something, but since in Oracle readers don't block writers and writers don't block readers, what exactly is the problem you are trying to solve?
From the perspective of the sessions that are reading the data, sessions that are doing inserts aren't really adding any overhead (updates might add a bit of overhead as the reader would have to look at data in the UNDO tablespace in order to reconstruct a read-consistent view of the data). From the perspective of the sessions that are inserting the data, sessions that are doing reads aren't really adding any overhead. Of course, your system as a whole might have a bottleneck that causes the various sessions to contend for resources (i.e. if your inserts are using up 100% of the available I/O bandwidth, that is going to slow down queries that have to do physical I/O), but that isn't directly related to the type of operations that the different sessions are doing-- you can flood an I/O subsystem with a bunch of reporting users just as easily as with a bunch of insert sessions.
You want transaction isolation read uncommitted. I don't recommend it but that's what you asked for :)
This will allow you to breach transaction isolation and read uncommitted inserted data.
Please read this Ask Tom article: http://www.oracle.com/technology/oramag/oracle/05-nov/o65asktom.html.
UPDATE: I was actually mistaking, Oracle doesn't really support read uncommitted isolation level, they just mention it :).
How about you try disabling all constraints in your table, then inserting all the data, then enabling them back again?
i.e. alter session set constraints=deffered;
However, if you had not set the constraints in your table to defferable during table creation, there might arise a slight problem.
What kind of performance volumes are you looking at? Are inserts batched or numerous small ones?
Before banging your head against the wall trying to think of clever ways to have good performance, did you create any simple prototypes which would give you a better picture of the out-of-the-box performance? It could easily turn out that you don't need to do anything special to meet the goals.

Resources