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.
Related
I am new to Oracle Exadata. My question is, to Index or not to Index in Exadata?
Found some of the blogs which says not to Index database Index and only to storage indexes which are temporary, but there are no official documentation from Oracle which says not to index in Exadata.
What are the issues if I index in Exadata? (since it is implemented in memory concepts), will it improve or downgrade performance? Is it better to drop index if already created?
We have huge datas 15 million plus and growing in Oracle Exadata with Varchars, CLOBS and other common datatypes. Not having any indexes created except primary keys. Why query is taking 10 to 12 minutes ( from 15 million records with simple select query having few where conditions) for execution? Oracle says Exadata is the fastest database in the planet.
The decision for an index is independent of the platform. It is always the same process, namely:
Does the benefits of having the index outweigh the cost of having the index.
Costs
has to be maintained
space overhead
might increase contention in high insert/update/delete frequency environments
Benefits
faster response times
The reason you might have less indexes in Exadata is that if other mechanisms (storage indexes, compression, flash, etc etc) can give you response times that meet your business requirements, then you can save on not having the drawbacks of those indexes.
But the decision process remains identical - cost vs benefit.
A common technique to assess an existing index is to make it invisible and see if there is an adverse (or beneficial) impact. In that way, if you have to revert and keep the index, there is no cost in doing so.
In addition to Connor's answer, be aware that an index in not always the best way to access the data. This is true even on non-Exadata storage systems. The process and considerations of whether to use an index is independent of Exadata; what Exadata does is give more reasons/capabilities not to use an index.
The oaktable article (shown in earlier comment) shows why it is better in exadata to most always not have an index. The note from Oracle below explains why. In a non-exadata DB dumb-storage return blocks (usually 8k) not rows, so for large tables a FTS is almost always a bad thing (unless you need most rows). Exadata has smart storage that has info from the query and tries to eliminate bytes that won't answer the query. It tries to return only bytes (not blocks) that may answer the query. This action lowers I/O back to the DB for processing. This way a FTS is not so bad and may actually be preferred. As a DBA, I have a DB with 12TB and many times I have to stick in a NO_INDEX hint to improve queries. This goes against normal modeling theory. Retrieving data from disk is the slowest process in the DB. Exadata removes unneeded data early in the process (at the storage level) and lessens the amount of data sent back to the DB for processing. Many times, my FTS on 8 billion row table is much faster than when using an index... only in Exadata ;)
http://www.oracle.com/technetwork/testcontent/o31exadata-354069.html
Oracle states that 'Invisible indexes are especially useful for testing the removal of an index before dropping it or using indexes temporarily without affecting the overall application.'
I don't understand why visibility is 'especially' useful for this, wouldn't making an index unusable be especially useful since DML operations are not maintained therefor it resembles dropping an index more so than making it simply invisible. I've never actually worked with this, I'm guessing that making an index invisible/visible is easier than making it usable/unusable because you have to rebuild an index somehow when you make it usable?
It is referring to the impact on your queries via statistics and the optimizer.
Many Oracle databases have complex schemas, user bases, as well as really large tables and indexes. Some even have very controlled schema statistics. Dropping a big index can be an expensive step (time-wise).
The statistics gatherer collects statistics to populate the data dictionary, which is used for the optimizer. Index stats are one of the key inputs to the Cost Based Optimizer. "Faking" the drop will cause the optimizer to act as if the index is gone, and you can then see the impact on the query plans. If you find that the drop wasn't such a good idea, you can immediately revert it. On the other hand, some indexes take hours to build, so you can see how it is valuable to be able to test it out first.
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
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.
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.