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We have a mid-size SQL Server based application that has no indexes defined. Not even on the the identity columns. I suggested to our moderately expensive application consultant that perhaps we might get better performance (particularly as our database grows) by creating some indexes on appropriate fields, and he said:
"Indexes will significantly impact other areas of the application and customers should not create them under any circumstances."
Anybody ever heard of anything like this? Are there ever circumstances where one should not create any indexes? I can see nothing special about this app - it's got int identity columns, then lots of string columns, bunch of relational tables but nothing special or weird that I can see.
Thanks!
[EDIT: the identity columns are not using "identity specification", they seem to be set by the program, looking at the database with Management Studio, I can find NO indexes...]
FOLLOWUP: At a conference I asked the CEO (and chief architect) of the company producing this product about this, his response was that they felt for small to midsize deployments, the overhead associated with maintaining indexes would have more of a negative to overall user experience (the application does a lot of writes) than the benefits of the indexes would offset, but for large databases, they do create indexes. The tech support guy was just overzealous and very unhelpful with his answer. Mystery solved.
Hire me and I'll create the indexes for you. 14 years' Sybase/SQL Server experience tells me to create those !darn! indexes. Unless your table has less than 500 records each.
My idea is that an index hash node is roughly sized to 1000.
The other thing you need to look out for is whether your consultant has normalized the tables. Perhaps, the table has 500 fields/columns, containing more than one conceptual entity or a whole dozen of conceptual entities. And that could be why he is nervous about creating indexes, because if there are 12 conceptual entities in the table there would be at least 12 set of indexes - in which case, he is absolutely true - under no circumstances ... blah blah.
However, if he indeed does have 500 columns or detectably multiple conceptual entities per table - he is a very very lousy data design engineer. In all my years working with more experienced data engineers, our tables rarely exceed 20 columns. 5 on the low side, 10 on the average. Sometimes for performance' sake we do allow mixing two entities in a table, or horizontalizing row occurrences into columns of a table.
When you look at the table design you can with an untrained eye see Product, Project, BuildSheet, FloorPlan, Equipment, etc records all rolled into one long row. You cannot mix all these entities together into one table.
That is the only reason I know why he could advise you against having indexes. If he is doing that, you should know that he is fraudulently representing his data design skills to your company and you should immediately drop him from your weekly contractual expenses.
OK, after reading larry's post - I agree with him too.
There is such a thing as over-indexing, especially in INSERT and UPDATE heavy applications with very large tables. So the answer to the question in your title is yes, it can sometimes be a bad idea to add indexes.
That's quite a different question from the one you ask in the body of your question, which is "Is it ever normal to have NO indexes in a SQL Server database". The answer is that unless you're using the database as a "write-only" system, in which data is added but only read after being bulk extracted and transformed into a another data store, it's exceedingly unusual not to have some indexes in the database.
Your consultant's statement is odd enough to make me believe that you may have left some important information out of your description. If not, I'd say he's nuts.
Do you have the disk space to spare? I've seen cases where the indexes weighed more than the table.
However, No indexes exist whatsoever! There can't be a case for that except for when all read operations need the entire table.
Columns with key constraints will have an implicit index on them anyway. So if you're always selecting by the primary key, then there's no point adding more indexes. If you're selecting by other criteria, then it makes sense to add indexes on those columns that you're querying on.
It also depends on how insert-heavy your data is. If you're inserting more often than you're querying, then the overhead of keeping the indexes up to date can make your inserts slower.
But to say you "should not create [indexes] under any circumstances" is a bit much.
What I would recommend is that you run the SQL Server Profiler tool with some your queries. This tool will recommend which indexes to add that will have the biggest effect on performance.
In most run-of-the-mill applications, the impact of indexes on insertion performance is a bit of non-issue. You're usually better off creating the index and if insertion performance drops dramatically (which it probably won't) you can try something else. Obviously there are some exceptions, where you should be more careful, like tables that are used for logging for instance.
As mentioned, disk space can be an issue.
Creating irrelevant indexes (e.g. duplicates) will also waste microseconds and occasionally result in a bad query execution plan.
The other problem I've seen is with strangely code third-party applications that generate parts of the database at runtime, and can delete or choke on indexes that they don't know about.
In the vast majority of cases though, a carefully chosen index will only be a benefit.
Not having indexes on id columns sounds really unusual and I would find any justification for not including them to smell very fishy.
You should be aware that if you are doing a high volume of commits to the database, adding more indexes will affect the speed of insertion, but no index on id? Wow.
It would be good to get better justification of exactly how adding extra indexes might cause problems though.
the more indexes you have the slower data inserts and modifications will be. Make sure that you add indexes when appropriate and write queries that can take advantage of those indexes, also if the selectivity leve of your index is low, it will not be used effectively
I would say that if your server is having troubles with CPU time, indexes could be a solution. If you are querying tables without indexes, the server will need a lot more resources and if tables are having millions of records, it can become a serious problem. I recently cooled down a CPU from 80-90% all the time to 10-20% just by putting the right indexes.
If using MS SQL, you could check the activity monitor to see what queries are expensive and create indexes based on the where clauses or joins.
Then at the recent expensive queries:
You can then right click and check the complete query!
For a few different reasons one of my projects is hosted on a shared hosting server
and developed in asp.Net/C# with access databases (Not a choice so don't laugh at this limitation, it's not from me).
Most of my queries are on the last few records of the databases they are querying.
My question is in 2 parts:
1- Is the order of the records in the database only visual or is there an actual difference internally. More specifically, the reason I ask is that the way it is currently designed all records (for all databases in this project) are ordered by a row identifying key (which is an auto number field) ascending but since over 80% of my queries will be querying fields that should be towards the end of the table would it increase the query performance if I set the table to showing the most recent record at the top instead of at the end?
2- Are there any other performance tuning that can be done to help with access tables?
"Access" and "performance" is an euphemism but the database type wasn't a choice
and so far it hasn't proven to be a big problem but if I can help the performance
I would sure like to do whatever I can.
Thanks.
Edit:
No, I'm not currently experiencing issues with my current setup, just trying to look forward and optimize everything.
Yes, I do have indexes and have a primary key (automatically indexes) on the unique record identifier for each of my tables. I definitely should have mentioned that.
You're all saying the same thing, I'm already doing all that can be done for access performance. I'll give the question "accepted answer" to the one that was the fastest to answer.
Thanks everyone.
As far as I know...
1 - That change would just be visual. There'd be no impact.
2 - Make sure your fields are indexed. If the fields you are querying on are unique, then make sure you make the fields a unique key.
Yes there is an actual order to the records in the database. Setting the defaults on the table preference isn't going to change that.
I would ensure there are indexes on all your where clause columns. This is a rule of thumb. It would rarely be optimal, but you would have to do workload testing against different database setups to prove the most optimal solution.
I work daily with legacy access system that can be reasonably fast with concurrent users, but only for smallish number of users.
You can use indexes on the fields you search for (aren't you already?).
http://www.google.com.br/search?q=microsoft+access+indexes
The order is most likely not the problem. Besides, I don't think you can really change it in Access anyway.
What is important is how you are accessing those records. Are you accessing them directly by the record ID? Whatever criteria you use to find the data you need, you should have an appropriate index defined.
By default, there will only be an index on the primary key column, so if you're using any other column (or combination of columns), you should create one or more indexes.
Don't just create an index on every column though. More indexes means Access will need to maintain them all when a new record is inserted or updated, which makes it slower.
Here's one article about indexes in Access.
Have a look at the field or fields you're using to query your data and make sure you have an index on those fields. If it's the same as SQL server you won't need to include the primary key in the index (assuming it's clustering on this) as it's included by default.
If you're running queries on a small sub-set of fields you could get your index to be a 'covering' index by including all the fields required, there's a space trade-off here, so I really only recommend it for 5 fields or less, depending on your requirements.
Are you actually experiencing a performance problem now or is this just a general optimization question? Also from your post it sounds like you are talking about a db with 1 table, is that accurate? If you are already experiencing a problem and you are dealing with concurrent access, some answers might be:
1) indexing fields used in where clauses (mentioned already)
2) Splitting tables. For example, if only 80% of your table rows are not accessed (as implied in your question), create an archive table for older records. Or, if the bulk of your performance hits are from reads (complicated reports) and you don't want to impinge on performance for people adding records, create a separate reporting table structure and query off of that.
3) If this is a reporting scenario, all queries are similar or the same, concurrency is somewhat high (very relative number given Access) and the data is not extremely volatile, consider persisting the data to a file that can be periodically updated, thus offloading the querying workload from the Access engine.
In regard to table order, Jet/ACE writes the actual table date in PK order. If you want a different order, change the PK.
But this oughtn't be a significant issue.
Indexes on the fields other than the PK that you sort on should make sorting pretty fast. I have apps with 100s of thousands of records that return subsets of data in non-PK sorted order more-or-less instantaneously.
I think you're engaging in "premature optimization," worrying about something before you actually have an issue.
The only circumstances in which I think you'd have a performance problem is if you had a table of 100s of thousands of records and you were trying to present the whole thing to the end user. That would be a phenomenally user-hostile thing to do, so I don't think it's something you should be worrying about.
If it really is a concern, then you should consider changing your PK from the Autonumber to a natural key (though that can be problematic, given real-world data and the prohibition on non-Null fields in compound unique indexes).
I've got a couple of things to add that I didn't notice being mentioned here, at least not explicitly:
Field Length, create your fields as large as you'll need them but don't go over - for instance, if you have a number field and the value will never be over 1000 (for the sake of argument) then don't type it as a Long Integer, something smaller like Integer would be more appropriate, or use a single instead of a double for decimal numbers, etc. By the same token, if you have a text field that won't have more than 50 chars, don't set it up for 255, etc, etc. Sounds obvious, but it's done, often times with the idea that "I might need that space in the future" and your app suffers in the mean time.
Not to beat the indexing thing to death...but, tables that you're joining together in your queries should have relationships established, this will create indexes on the foreign keys which greatly increases the performance of table joins (NOTE: Double check any foreign keys to make sure they did indeed get indexed, I've seen cases where they haven't been - so apparently a relationship doesn't explicitly mean that the proper indexes have been created)
Apparently compacting your DB regularly can help performance as well, this reduces internal fragmentation of the file and can speed things up that way.
Access actually has a Performance Analyzer, under tools Analyze > Performance, it might be worth running it on your tables & queries at least to see what it comes up with. The table analyzer (available from the same menu) can help you split out tables with alot of redundant data, obviously, use with caution - but it's could be helpful.
This link has a bunch of stuff on access performance optimization on pretty much all aspects of the database, tables, queries, forms, etc - it'd be worth checking out for sure.
http://office.microsoft.com/en-us/access/hp051874531033.aspx
To understand the answers here it is useful to consider how access works, in an un-indexed table there is unlikely to be any value in organising the data so that recently accessed records are at the end. Indeed by the virtue of the fact that Access / the JET engine is an ISAM database it's the other way around. (http://en.wikipedia.org/wiki/ISAM) That's rather moot however as I would never suggest putting frequently accessed values at the top of a table, it is best as others have said to rely on useful indexes.
I have an app, which cycles through a huge number of records in a database table and performs a number of SQL and .Net operations on records within that database (currently I am using Castle.ActiveRecord on PostgreSQL).
I added some basic btree indexes on a couple of the feilds, and as you would expect, the performance of the SQL operations increased substantially. Wanting to make the most of dbms performance I want to make some better educated choices about what I should index on all my projects.
I understand that there is a detrement to performance when doing inserts (as the database needs to update the index, as well as the data), but what suggestions and best practices should I consider with creating database indexes? How do I best select the feilds/combination of fields for a set of database indexes (rules of thumb)?
Also, how do I best select which index to use as a clustered index? And when it comes to the access method, under what conditions should I use a btree over a hash or a gist or a gin (what are they anyway?).
Some of my rules of thumb:
Index ALL primary keys (I think most RDBMS do this when the table is created).
Index ALL foreign key columns.
Create more indexes ONLY if:
Queries are slow.
You know the data volume is going to increase significantly.
Run statistics when populating a lot of data in tables.
If a query is slow, look at the execution plan and:
If the query for a table only uses a few columns, put all those columns into an index, then you can help the RDBMS to only use the index.
Don't waste resources indexing tiny tables (hundreds of records).
Index multiple columns in order from high cardinality to less. This means: first index the columns with more distinct values, followed by columns with fewer distinct values.
If a query needs to access more than 10% of the data, a full scan is normally better than an index.
Here's a slightly simplistic overview: it's certainly true that there is an overhead to data modifications due to the presence of indexes, but you ought to consider the relative number of reads and writes to the data. In general the number of reads is far higher than the number of writes, and you should take that into account when defining an indexing strategy.
When it comes to which columns to index I'v e always felt that the designer ought to know the business well enough to be able to take a very good first pass at which columns are likely to benefit. Other then that it really comes down to feedback from the programmers, full-scale testing, and system monitoring (preferably with extensive internal metrics on performance to capture long-running operations),
As #David Aldridge mentioned, the majority of databases perform many more reads than they do writes and in addition, appropriate indexes will often be utilised even when performing INSERTS (to determine the correct place to INSERT).
The critical indexes under an unknown production workload are often hard to guess/estimate, and a set of indexes should not be viewed as set once and forget. Indexes should be monitored and altered with changing workloads (that new killer report, for instance).
Nothing beats profiling; if you guess your indexes, you will often miss the really important ones.
As a general rule, if I have little idea how the database will be queried, then I will create indexes on all Foriegn Keys, profile under a workload (think UAT release) and remove those that are not being used, as well as creating important missing indexes.
Also, make sure that a scheduled index maintenance plan is also created.
Are there any hard limits on the number of rows in a table in a sql server table? I am under the impression the only limit is based around physical storage.
At what point does performance significantly degrade, if at all, on tables with and without an index. Are there any common practicies for very large tables?
To give a little domain knowledge, we are considering usage of an Audit table which will log changes to fields for all tables in a database and are wondering what types of walls we might run up against.
You are correct that the number of rows is limited by your available storage.
It is hard to give any numbers as it very much depends on your server hardware, configuration, and how efficient your queries are.
For example, a simple select statement will run faster and show less degradation than a Full Text or Proximity search as the number of rows grows.
BrianV is correct. It's hard to give a rule because it varies drastically based on how you will use the table, how it's indexed, the actual columns in the table, etc.
As to common practices... for very large tables you might consider partitioning. This could be especially useful if you find that for your log you typically only care about changes in the last 1 month (or 1 day, 1 week, 1 year, whatever). You could then archive off older portions of the data so that it's available if absolutely needed, but won't be in the way since you will almost never actually need it.
Another thing to consider is to have a separate change log table for each of your actual tables if you aren't already planning to do that. Using a single log table makes it VERY difficult to work with. You usually have to log the information in a free-form text field which is difficult to query and process against. Also, it's difficult to look at data if you have a row for each column that has been changed because you have to do a lot of joins to look at changes that occur at the same time side by side.
In addition to all the above, which are great reccomendations I thought I would give a bit more context on the index/performance point.
As mentioned above, it is not possible to give a performance number as depending on the quality and number of your indexes the performance will differ. It is also dependent on what operations you want to optimize. Do you need to optimize inserts? or are you more concerned about query response?
If you are truly concerned about insert speed, partitioning, as well a VERY careful index consideration is going to be key.
The separate table reccomendation of Tom H is also a good idea.
With audit tables another approach is to archive the data once a month (or week depending on how much data you put in it) or so. That way if you need to recreate some recent changes with the fresh data, it can be done against smaller tables and thus more quickly (recovering from audit tables is almost always an urgent task I've found!). But you still have the data avialable in case you ever need to go back farther in time.
I'm working on a project with a rather large Oracle database (although my question applies equally well to other databases). We have a web interface which allows users to search on almost any possible combination of fields.
To make these searches go fast, we're adding indexes to the fields and combinations of fields on which we believe users will commonly search. However, since we don't really know how our customers will use this software, it's hard to tell which indexes to create.
Space isn't a concern; we have a 4 terabyte RAID drive of which we are using only a small fraction. However, I'm worried about the possible performance penalties of having too many indexes. Because those indexes need to be updated every time a row is added, deleted, or modified, I imagine it'd be a bad idea to have dozens of indexes on a single table.
So how many indexes is considered too many? 10? 25? 50? Or should I just cover the really, really common and obvious cases and ignore everything else?
It depends on the operations that occur on the table.
If there's lots of SELECTs and very few changes, index all you like.... these will (potentially) speed the SELECT statements up.
If the table is heavily hit by UPDATEs, INSERTs + DELETEs ... these will be very slow with lots of indexes since they all need to be modified each time one of these operations takes place
Having said that, you can clearly add a lot of pointless indexes to a table that won't do anything. Adding B-Tree indexes to a column with 2 distinct values will be pointless since it doesn't add anything in terms of looking the data up. The more unique the values in a column, the more it will benefit from an index.
I usually proceed like this.
Get a log of the real queries run on the data on a typical day.
Add indexes so the most important queries hit the indexes in their execution plan.
Try to avoid indexing fields that have a lot of updates or inserts
After a few indexes, get a new log and repeat.
As with all any optimization, I stop when the requested performance is reached (this obviously implies that point 0. would be getting specific performance requirements).
Everyone else has been giving you great advice. I have an added suggestion for you as you move forward. At some point you have to make a decision as to your best indexing strategy. In the end though, the best PLANNED indexing strategy can still end up creating indexes that don't end up getting used. One strategy that lets you find indexes that aren't used is to monitor index usage. You do this as follows:-
alter index my_index_name monitoring usage;
You can then monitor whether the index is used or not from that point forward by querying v$object_usage. Information on this can be found in the Oracle® Database Administrator's Guide.
Just remember that if you have a warehousing strategy of dropping indexes before updating a table, then recreating them, you will have to set the index up for monitoring again, and you'll lose any monitoring history for that index.
In data warehousing it is very common to have a high number of indexes. I have worked with fact tables having two hundred columns and 190 of them indexed.
Although there is an overhead to this it must be understood in the context that in a data warehouse we generally only insert a row once, we never update it, but it can then participate in thousands of SELECT queries which might benefit from indexing on any of the columns.
For maximum flexibility a data warehouse generally uses single column bitmap indexes except on high cardinality columns, where (compressed) btree indexes can be used.
The overhead on index maintenance is mostly associated with the expense of writing to a great many blocks and the block splits as new rows are added with values that are "in the middle" of existing value ranges for that column. This can be mitigated by partitioning and having the new data loads aligned with the partitioning scheme, and by using direct path inserts.
To address your question more directly, I think it is probably fine to index the obvious at first, but do not be afraid of adding more indexes on if the queries against the table would benefit.
In a paraphrase of Einstein about simplicity, add as many indexes as you need and no more.
Seriously, however, every index you add requires maintenance whenever data is added to the table. On tables that are primarily read only, lots of indexes are a good thing. On tables that are highly dynamic, fewer is better.
My advice is to cover the common and obvious cases and then, as you encounter issues where you need more speed in getting data from specific tables, evaluate and add indices at that point.
Also, it's a good idea to re-evaluate your indexing schemes every few months, just to see if there is anything new that needs indexing or any indices that you've created that aren't being used for anything and should be gotten rid of.
In addition to the points everyone else has raised, the Cost Based Optimizer incurs a cost when creating a plan for an SQL statement if there are more indexes because there are more combinations for it to consider. You can reduce this by correctly using bind variables so that SQL statements stay in the SQL cache. Oracle can then do a soft parse and re-use the plan it found last time.
As always, nothing is simple. If there are skewed columns and histograms involved then this can be a bad idea.
In our web applications we tend to limit the combinations of searches that we allow. Otherwise you would have to test literally every combination for performance to ensure you did not have a lurking problem that someone will find one day. We have also implemented resource limits to stop this causing issues elsewhere in the application should something go wrong.
I made some simple tests on my real project and real MySql database. I already answered in this topic: What is the cost of indexing multiple db columns?
But I think it will be better if I quote it here:
I made some simple tests using my real
project and real MySql database.
My results are: adding average index
(1-3 columns in an index) to a table -
makes inserts slower by 2.1%. So, if
you add 20 indexes, your inserts will
be slower by 40-50%. But your selects
will be 10-100 times faster.
So is it ok to add many indexes? - It
depends :) I gave you my results - You
decide!
Ultimately how many indexes you need depend on the behavior of your applications that ride on top of your database server.
In general the more inserting you do the more painful your indexes become. Each time you do an insert, all the indexes that include that table have to be updated.
Now if your application has a decent amount of reading, or even more so if it's almost all reading, then indexes are the way to go as there will be major performance improvements for very little cost.
There's no static answer in my opinion, this sort of thing falls under 'performance tuning'.
It could be that everything your app does is looked up by a primary key, or it could be the oposite in that queries are done over unristricted combinations of fields and any one in particular could be used at any given time.
Beyond just indexing, there's reogranizing your DB to include calculated search fields, splitting tables, etc - it's really dependant on your load shapes and query parameters, how much/what data 'really' needs to be retruend by a query.
If your entire DB is fronted by stored-procedure facades turning becomes a bit easier, as you don't have to wory about every ad-hoc query. Or you may have a deep understanding of the kind of queries that will hit your DB, and can limit the tuning to those.
For SQL Server I've found the Database Engine Tuning advisor usefull - you set up 'typical' workloads and it can make recommendations about adding/removing indexes and statistics. I'm sure other DBs have similar tools, either 'offical' or third party.
This really is a more theoretical questions than practical. Indexes impact on your performance depends on the hardware you have, the version of Oracle, index types, etc. Yesterday I heard Oracle announced a dedicated storage, made by HP, which is supposed to perform 10 times faster with 11g database.
As for your case, there can be several solutions:
1. Have a large amount of indexes (>20) and rebuild them daily (nightly). This would be especially useful if the table gets thousands of updates/deletes daily.
2. Partition your table (if that applies your data model).
3. Use a separate table for new/updated data, and run a nightly process which combines the data together. This would require a change in your application logic.
4. Switch to IOT (index organized table), if your data support this.
Of course there might be many more solutions for such case. My first suggestion to you, would be to clone the DB to a development environment, and run some stress testing against it.
An index imposes a cost when the underlying table is updated. An index provides a benefit when it is used to spped up a query. For each index, you need to balance the cost against the benefit. How much slower does the query run without the index? How much of a benefit is running faster? Can you or your users tolerate the slow speed when the index is missing?
Can you tolerate the additional time it takes to complete an update?
You need to compare costs and benefits. That's particular to your situation. There's no magic number of indexes that passes the threshold of "too many".
There's also the cost of the space needed to store the index, but you've said that in your situation that's not an issue. The same is true in most situations, given how cheap disk space has become.
If you do mostly reads (and few updates) then there's really no reason not to index everything you'll need to index. If you update often, then you may need to be cautious on how many indexes you have. There's no hard number, but you'll notice when things start to slow down. Make sure your clustered index is the one that makes the most sense based on the data.
One thing you may consider is building indexes to target a standard combination of searches. If column1 is commonly searched, and column2 is often used with it, and column3 is sometimes used with column2 and column1, then an index on column1, column2, and column3 in that order can be used for any of those three circumstances, though it is only one index that has to be maintained.
How many columns are there?
I have always been told to make single-column indexes, not multi-column indexes. So no more indexes than the amount of columns, IMHO.
What it really comes down to is, don't add an index unless you know (and this often means gathering usage statistics) that it will be used far more often than it's updated.
Any index that doesn't meet that criteria will cost you more to rebuild than the performance penalty of not having it in the odd case it got used.
Sql server gives you some good tools that let you see which indexes are actually being used.
This article, http://www.mssqltips.com/tip.asp?tip=1239, gives you some queries that let you get a better insight into how much an index is used, as opposed to how much it is updated.
It is totally based on the columns which are being used in Where Clause.
And as the Thumb of Rule, we must have indexes on Foreign Key Columns to avoid DEADLOCKS.
AWR report should analyze periodically to understand the need of indexes.