Is this true that Update SQL Query is slow because of Clustered index??????
You would be better off saying 'slower' rather than 'slow'. When data is written to a clustered index, and it doesn't go at the very end of the table, data needs to be joggled around in order to fit it in, in the same way that adding a CD into a big stack of alphabetised CD is a lot slower than just sticking it on the top.
If you don't have any clustered indexes at all, then what you have is termed a "heap". You also have a heap of trouble, since the order of the data in your table is random - and selecting data from the table will be slow. That may be OK if you're doing many more INSERTs than you are SELECTs, but usually that's not the case.
Whether the clustered index makes INSERTs slower or not depends on:
The fill factor of the table (i.e. whether there are enough gaps in the data to allow new data to be inserted without moving everything around).
What columns are chosen as the cluster key.
If you're using an identity column as the cluster key, then you may find that insert performance is perfectly fine, since new entries are always being added on the end. The same may apply to a datetime column if using the current date (which of course also keeps increasing).
You need to keep the size of the cluster key small, since that's the index into the data that's stored in every other index. For example, if your cluster key consists of 3 ints and a datetime, then each entry in all your other indexes will include all that data in addition to whatever it was that you tried to index. For this reason, an identity column is actually a pretty good choice of cluster key since it's nice & small.
The perfect cluster key in any situation can only be chosen with a good deal of thought and a lot of testing (with realistically large data sets). Having a good cluster key can make a huge difference to SELECT performance - which normally outweighs any degradation in INSERT performance.
Define slow, ofcourse the clustered index will always be slower than a non-clustered index...
Insertion and updates are slower because of clustered indexes (particularly on huge tables) - but selects are way faster.
Making the index non-clustered usually improves inserts and updates performance retaining selection performance (selects are often less performant with a non-clustered index compared with a clustered index but something's gotta give).
A clustered index dictates how a table is physically stored on disk, and so updating a table with a clustered index may require that significant parts of the table be moved to make space for the new record, and that's slow.
You can mitigate the problem by setting an appropriate fillfactor for your indexes. It's not quite so bad that you have to re-jigger the whole table when you add a record to the middle; it's usually just a few pages. Fillfactor determines how much of each page is filled before creating a new page, and how much to leave as wiggle room for new insertions. A lower fillfactor on an index will leave more space for new records and therefore give faster insert times on average, at the cost of more disk space and more pages and therefore slower reads. But if you're doing a lot more updating than reading it may be worth it.
Related
I have just taken over a database which has around 2200 tables. Over 2000 of these have no clustered index (some have no indexes at all).
All of the tables have been configured to use a GUID as the uniqueidentifier.
Just looking at the query plans, I can see that there are many table scans occurring. Most searches use the uniqueidentifier to search on.
I am wondering if it is better to have a clustered index on the GUID than not to have a clustered index at all. I imagine that a clustered index on a 16-byte column will inevitably lead to fragmentation.
I could arguably cluster on other columns but the majority of searches tend to search by or join via the GUIDS.
Any advice would be very much welcomed. I've never seen so many GUID's!!
In generally, I would recommend having an identity column as the primary key and use that for clustering. This is also a better choice for joins.
Why? First, identity keys are generally shorter that unique ids. So, foreign key references and indexes are smaller.
More importantly, inserts would always go at the "end" of the table. When using GUIDs, inserts are often going to cause fragmentation. If you are inserting rows, I would say that a secondary index on the GUID might be better than a clustered index (the fragmentation is only in the index).
With 2000 tables, I doubt you will change the structure. You can ameliorate the fragmentation using newsequentialid().
GUID column with random values usually is not the best choice for a clustered index because it could be the root cause of an index fragmentation:
Read ahead opportunity of the database won't be effective;
The cost of insert operations will be too expensive, because in this case you'll got lots of page split overhead;
There are 3 ways how you can live with that:
Schedule planning index reorganizing and rebuilding which will reduce index fragmentation and improve your statistics automatically;
Use
newsequantialid for generating values of this column;
Generate
GUID value sequantialy outside of the database (Guid.Comb
Identifier is a great example of solving this issue in
NHibernate).
This is a really a comment on your question to Gordon's good answer:
Firstly, don't forget to check the index DMVs to see which ones are being used (or not used) and have a look at the expensive query plans in the cache to focus on the tables and queries that will be causing most pain. I would expect that many of those 2200 tables are relatively small & the queries are able to look up pretty quickly even from the guid clustered index.
For those tables that aren't clustered, clustering on the guid would reduce fragmentation, since it forces all the data for the table to be colocated rather than allowing pages to be put in the next free extent & spreading tables all over the disk. This should make some of the I/O more efficient.
Check you have a low enough fillfactor so that your regular index rebuilds avoid page splitting in advance, although it will also be workload dependent (OLTP vs DW and read/write ratio of table)
If you have applications that are doing explicit column selects/inserts then you may be able to add an identity column without breaking anything. That allows you cluster around the identity & add an index to the guid. Whether this really helps depends on the relative (in)efficency of the new plans.
You could consider clustering around a non-guid field where queries will lookup against it fairly regularly (eg, a date range) and index the guid separately.
You'd have to look at the queries & relative performance for that more closely.
I have a DB containing some tables, no table has non-clustered index defined. The big application which uses this DB is slow(because the number of rows are close to a million). I want to optimize DB fetch operations by adding indexes. When I read about indexes I came across index names like:
Clustered Index
Non clustered Index on a Clustered Index
Non Clustered Index on a heap
Also, indexes need to be created only on some columns. How will I identify that in a table which kind of index need to be created and across which column(s)?
P.S. Execution plan while running query tells to create NCI on all columns. Can I blindly go ahead and create index as suggested by SQL Server?
A clustered index is a type of index which defines how the data of your table will be stored (more precisely, how the data is sorted). This is the reason why the clustered index columns should be chosen very carefully (sequentially inserted data is primordial or you will end up with fragmentation and performance issues over time, an integer "identity" column is a good pick for example).
I found out that it is a good practice to always have a clustered index on your permanent tables.
A table without a clustered index is a heap because data is not sorted in a particular way (it'll be added at the end of the file), data is therefore harder to retrieve. The only improvement you can get from using a heap without indexes is that data insertion will be faster.
A non-clustered index is a separate file that will help speed up your queries on the columns you choose (it will store values of the indexed data and their reference to the location in the main file). As the data of your table become more and more important, having those separate files can dramatically improve the performance of your queries because the db engine won't have to scan the entire table for the data you are looking for, but just look for the position of the rows to retrieve in the index file (which contains ordered data of the columns you've chosen).
Adding indexes will speed up your select queries, but slow down writing operations as the indexes have to be updated. So, don't create too many indexes on too many columns !
There are two types of tables: heap tables (which have no clustered index) and clustered tables (which do). Each of these can have any number of non-clustered indexes built on them.
When do you use a heap table? Realistically, in only one scenario: when you're doing parallel bulk imports. This specific scenario requires that the table have no clustered index. In all other scenarios, a heap table has worse performance than a table with a clustered index -- don't take my word for it, though: Microsoft has an article on this that, while dated, is still relevant. In other words, for most practical database work, you can ignore heap tables as a curiosity.
On what do you create your clustered index? Ideally, on a column with values that are ever increasing (or decreasing) and aren't changed in updates. Why? Because this has the least overhead for updating, as no data has to be moved. Because of these two requirements, surrogate keys in the form of IDENTITY columns are popular, since they neatly meet them. This is certainly not the only possible choice, though: indexing on an ever increasing timestamp is also popular (in big data warehouses, for example).
With that (mostly) out of the way, how do you decide what other columns to index? Now that's a great question, but not one I feel qualified to answer in all its glory here. I've gotten a lot of experience myself with index design over the years, but I'm not aware of specific books or articles that I could recommend (which is not to say they don't exist, and I hope other people can chime in with suggestions). For what it's worth, Microsoft itself has written a guide here, which is quite in-depth (perhaps too much so), but I haven't thoroughly read this myself.
Can you blindly go ahead and create the indexes as suggested by the query optimizer? If by that you mean "should I", then the answer is almost certainly no. The query optimizer is very eager to suggest and and all possible indexes that could speed up a query, but that doesn't mean they should all be created -- every index increases the overhead of performing inserts and updates on the table. If you followed the optimizer's advice, it's probable that you would eventually end up with indexes covering every possible combination of columns, which would be pretty terrible for anything that's not a SELECT query. Having said that, creating too many indexes is almost always not as awful as creating no indexes at all, since that quickly kills performance for most queries that involve tables with more than about 10.000 rows.
I could write books on this topic, but I haven't the time or (I fear) the skill. I hope this at least gets you started.
I recently became involved with a new software project which uses SQL Server 2000 for its data storage.
In reviewing the project, I discovered that one of the main tables uses a clustered index on its primary key which consists of four columns:
Sequence numeric(18, 0)
Date datetime
Client varchar(9)
Hash tinyint
This table experiences a lot of inserts in the course of normal operation.
Now, I'm a C++ developer, not a DB Admin, but my first impression of this table design was that that having these fields as a clustered index would be very detrimental to insert performance, since the data would have to be physically reordered on each insert.
In addition, I can't really see any benefit to this since one would have to be querying all of these fields frequently to justify the clustered index, right?
So basically I need some ammunition for when I go to the powers that be to convince them that the table design should be changed.
The clustered index should contain the column(s) most queried by to give the greatest chance of seeks or of making a nonclustered index cover all the columns in the query.
The primary key and the clustered index do not have to be the same. They are both candidate keys, and tables often have more than one such key.
You said
In addition, I can't really see any benefit to this since one would have to be querying all of these fields frequently to justify the clustered index, right?
That's not true. A seek can be had just by using the first column or two of the clustered index. It may be a range seek, but it's still a seek. You don't have to specify all the columns of it in order to get that benefit. But the order of the columns does matter a lot. If you're predominantly querying on Client, then the Sequence column is a bad choice as the first in the clustered index. The choice of the second column should be the item that is most queried in conjunction with the first (not by itself). If you find that a second column is queried by itself almost as often as the first column, then a nonclustered index will help.
As others have said, reducing the number of columns/bytes in the clustered index as much as possible is important.
It's too bad that the Sequence is a random value instead of incrementing, but that may not be able to be helped. The answer isn't to throw in an identity column unless your application can start using it as the primary query condition on this table (unlikely). Now, since you're stuck with this random Sequence column (presuming it IS the most often queried), let's look at another of your statements:
having these fields as a clustered index would be very detrimental to insert performance, since the data would have to be physically reordered on each insert.
That's not entirely true.
The physical location on the disk is not really what we're talking about here, but it does come into play in terms of fragmentation, which is a performance implication.
The rows inside each 8k page are not ordered. It's just that all the rows in each page are less than the next page and more than the previous one. The problem occurs when you insert a row and the page is full: you get a page split. The engine has to copy all the rows after the inserted row to a new page, and this can be expensive. With a random key you're going to get a lot of page splits. You can ameliorate the problem by using a lower fillfactor when rebuilding the index. You'd have to play with it to get the right number, but 70% or 60% might serve you better than 90%.
I believe that having datetime as the second CI column could be beneficial, since you'd still be dealing with pages needing to be split between two different Sequence values, but it's not nearly as bad as if the second column in the CI was also random, since you'd be guaranteed to page split on every insert, where with an ascending value you can get lucky if the row can be added to a page because the next Sequence number starts on the next page.
Shortening the data types and number of all columns in a table as well as its nonclustered indexes can boost performance too, since more rows per page = fewer page reads per request. Especially if the engine is forced to do a table scan. Moving a bunch of rarely-queried columns to a separate 1-1 table could do wonders for some of your queries.
Last, there are some design tweaks that could help as well (in my opinion):
Change the Sequence column to a bigint to save a byte for every row (8 bytes instead of 9 for the numeric).
Use a lookup table for Client with a 4-byte int identity column instead of a varchar(9). This saves 5 bytes per row. If possible, use a smallint (-32768 to 32767) which is 2 bytes, an even greater savings of 7 bytes per row.
Summary: The CI should start with the column most queried on. Remove any columns from the CI that you can. Shorten columns (bytes) as much as you can. Use a lower fillfactor to mitigate the page splits caused by the random Sequence column (if it has to stay first because of being queried the most).
Oh, and get your online defragging going. If the table can't be changed, at least it can be reorganized frequently to keep it in best possible shape. Don't neglect statistics, either, so the engine can pick appropriate execution plans.
UPDATE
Another strategy to consider is if the composite key used in the table can be converted to an int, and a lookup table of the values is created. Let's say some combination of less than all 4 columns is repeated in over 100 rows, for example, Sequence + Client + Hash but only with varying Date values. Then an insert to a separate SequenceClientHash table with an identity column could make sense, because then you could look up the artificial key once and use it over and over again. This would also get your CI to add new rows only on the last page (yay) and substantially reduce the size of the CI as repeated in all nonclustered indexes (yippee). But this would only make sense in certain narrow usage patterns.
Now, marc_s suggested just adding an additional int identity column as the clustered index. It is possible that this could help by making all the nonclustered indexes get more rows per page, but it all depends on exactly where you want the performance to be, because this would guarantee that every single query on the table would have to use a bookmark lookup and you could never get a table seek.
About "tons of page splits and bad index fragmentation": as I already said this can be ameliorated somewhat with a lower fill factor. Also, frequent online index reorganization (not the same as rebuilding) can help reduce the effect of this.
Ultimately, it all comes down to the exact system and its unique pattern of data access combined with decisions about which parts you want optimized. For some systems, having a slower insert isn't bad as long as selects are always fast. For others, having consistent but slightly slower select times is more important than having slightly faster but inconsistent select times. For others, the data isn't really read until it's pushed to a data warehouse anyway so the inserts need to be as fast as possible. And adding into the mix is the fact that performance isn't just about user wait time or even query response time but also about server resources especially in the case of massive parallelism, so that total throughput (say, in client responses per time unit) matters more than any other factor.
Clustered indexes (CI) work best over ever-increasing, narrow, rarely changing values. You'll want your CI to cover the column(s) that get hit the most often in queries with >=, <=, or BETWEEN statements.
I'm not sure how your data normally gets hit. Most often you'll see a CI on an IDENTITY column or another narrow column (because this column will also be returned "tacked on" to all non-clustered indexes, and we don't want a ton of data added on to every fetch if it isn't needed). It's possible the data might be getting queried most often on date, and that may be a good choice, but all four columns is likely not correct (I stress likely, because I don't know the set-up; this may not have anything wrong with it). There are some pointers here: http://msdn.microsoft.com/en-us/library/aa933131%28SQL.80%29.aspx
There are a few things you are misunderstanding about how SQL creates and uses indexes.
Clustered indexes aren't necessarily physically ordered on disk by the clustered index, at least not in real-time. They are just a logical ordering.
I wouldn't expect a major performance hit based on this structure and removing the clustered index before you have actually identified a performance issue related to that index is clearly premature optimization.
Also, an index can be useful (especially one with several fields in it) even for searches that don't sort or get queried on all columns included in it.
Obviously, there should be a justification for creating a multi-part clustered index, just like any index, so it makes sense to ask for that if you think it was added capriciously.
Bottom line: Don't optimize the indexes for insert performance until you have actually detected a performance problem with inserts. It usually isn't worth it.
If you have only that single clustered index on your table, that might not be too bad. However, the clustering index is also used for looking up the real data page for any hit in a non-clustered index - therefor, the clustered index (all its columns) are also part of each and every non-clustered index you might have on your table.
So if you have a few nonclustered indices on your table, then you're definitely a) wasting a lot of space (and not just on disk - also in your server's RAM!), and b) your performance will be bad.
A good clustered index ought to be:
small (best bet: a 4-byte INT) - yours is pretty bad with up to 28 bytes per entry
unique
stable (never change)
ever-increasing
I would bet your current setup violates at least two if not more of those requirements. Not following these recommendations will lead to waste of space, and as you rightfully say, lots of page and index fragmentation and page splits (having to "rearrange" the data when an insert happens somewhere in the middle of the clustered index).
Quite honestly: just add a surrogate ID INT IDENTITY(1,1) to your table and make that the primary clustered key - you should see quite a nice boost in performance, just from that, if you have lots of INSERT (and UPDATE) operations going on!
See some more background info on what makes a good clustering key, and what is important about them, here:
GUIDs as PRIMARY KEYs and/or the clustering key
The Clustered Index Debate Continues...
Ever-increasing clustering key - the Clustered Index Debate..........again!
I ultimately agree with Erik's last paragraph:
"Ultimately, it all comes down to the exact system and its unique pattern of data access combined with decisions about which parts you want optimized..."
This is the basic thing I force people to learn: there's no universal solution.
You have to know your data and the actions performed against it. You have to know how frequent different type of actions are and their impact and expected execution times (you don't have to hard tune some rarely executed query and impact everything else if the end user agrees the query execution time is not so important--let's say waiting for few minutes for some report once per week is okay). Of course, as Erik said
"performance isn't just about user wait time or even query response time but also about server resources"
If such a query affects overall server performance, it should be considered as a serious candidate for optimization, even if execution time is fine. I've seen some very fast queries that used huge amount of CPU on multiprocessor servers, while slightly slower solution were incomparable "lighter" from resource utilization point of view. In that case I almost always go for the slower one.
Once you know what is your goal you can decide how many indexes you need and which one should be clustered. Unique constraints, filtered indexes, indexes with included columns are quite powerful tools for tuning. Choosing proper columns is important, but often choosing proper order of columns is even more important. And at the end, don't kill insert/update performance with tons of indexes if the table is frequently modified.
I'm not a DBA ("Good!", you'll be thinking in a moment.)
I have a table of logging data with these characteristics and usage patterns:
A datetime column for storing log timestamps whose value is ever-increasing and mostly (but only mostly) unique
Frequent-ish inserts (say, a dozen a minute), only at the end of the timestamp range (new data being logged)
Infrequent deletes, in bulk, from the beginning of the timestamp range (old data being cleared)
No updates at all
Frequent-ish selects using the timestamp column as the primary criterion, along with secondary criteria on other columns
Infrequent selects using other columns as the criteria (and not including the timestamp column)
A good amount of data, but nowhere near enough that I'm worried much about storage space
Additionally, there is currently a daily maintenance window during which I could do table optimization.
I frankly don't expect this table to challenge the server it's going to be on even if I mis-index it a bit, but nevertheless it seemed like a good opportunity to ask for some input on SQL Server clustered indexes.
I know that clustered indexes determine the storage of the actual table data (the data is stored in the leaf nodes of the index itself), and that non-clustered indexes are separate pointers into the data. So in query terms, a clustered index is going to be faster than a non-clustered index -- once we've found the index value, the data is right there. There are costs on insert and delete (and of course an update changing the clustered index column's value would be particularly costly).
But I read in this answer that deletes leave gaps that don't get cleaned up until/unless the index is rebuilt.
All of this suggests to me that I should:
Put a clustered index on the timestamp column with a 100% fill-factor
Put non-clustered indexes on any other column that may be used as a criterion in a query that doesn't also involve the clustered column (which may be any of them in my case)
Schedule the bulk deletes to occur during the daily maintenance interval
Schedule a rebuild of the clustered index to occur immediately after the bulk delete
Relax and get out more
Am I wildly off base there? Do I need to frequently rebuild the index like that to avoid lots of wasted space? Are there other obvious (to a DBA) things I should be doing?
Thanks in advance.
Contrary to what a lot of people believe, having a good clustered index on a table can actually make operations like INSERTs faster - yes, faster!
Check out the seminal blog post The Clustered Index Debate Continues.... by Kimberly Tripp - the ultimate indexing queen.
She mentions (about in the middle of the article):
Inserts are faster in a clustered
table (but only in the "right"
clustered table) than compared to a
heap. The primary problem here is that
lookups in the IAM/PFS to determine
the insert location in a heap are
slower than in a clustered table
(where insert location is known,
defined by the clustered key). Inserts
are faster when inserted into a table
where order is defined (CL) and where
that order is ever-increasing.
The crucial point is: only with the right clustered index will you be able to reap the benefits - when a clustered index is unique, narrow, stable and optimally ever-increasing. This is best served with an INT IDENTITY column.
Kimberly Tripp also has a great article on how to pick the best possible clustering key for your tables, and what criteria it should fulfil - see her post entitled Ever-increasing clustering key - the Clustered Index Debate..........again!
If you have such a column - e.g. a surrogate primary key - use that for your clustering key and you should see very nice performance on your table - even on lots of INSERTs.
I agree with putting the clustered index on the timestamp column. My query would be on the fillfactor - 100% gives best read performance at the expense of write performance. you may be hurt by page splits. Choosing a lower fillfactor will delay page splitting at the expense of read performance so its a fine balancing act to get the best for your situation.
After the bulk deletes its worth rebuilding the indexes and updating statistics. This not only keeps performance up but also resets the indexes to the specified fillfactor.
Finally, yes put nonclustered indexes on other appropriate columns but only ones that are very select e.g not bit fields. But remember the more indexes, the more this affects write performance
There's two "best practice" ways to index a high traffic logging table:
an integer identity column as a primary clustered key
a uniqueidentifier colum as primary key, with DEFAULT NEWSEQUENTIALID()
Both methods allow SQL Server to grow the table efficiently, because it knows that the index tree will grow in a particular direction.
I would not put any other indexes on the table, or schedule rebuilds of the index, unless there is a specific performance issue.
The obvious answer is it depends on how you will query it. The point of the index is to lessen the quantity of compares when selecting data. The clustered index helps when you consider what data you will load together and the blocking factor of the storage (you can load a bunch of data in a 64k block with one read). If you include an ID and a datetime as the primary key, but not use them in your selection criteria, they will do nothing but hinder your performance. This is why people usually drop indexes upon bulk inserts before loading data.
The table in question is part of a database that a vendor's software uses on our network. The table contains metadata about files. The schema of the table is as follows
Metadata
ResultID (PK, int, not null)
MappedFieldname (char(50), not null)
Fieldname (PK, char(50), not null)
Fieldvalue (text, null)
There is a clustered index on ResultID and Fieldname. This table typically contains millions of rows (in one case, it contains 500 million). The table is populated by 24 workers running 4 threads each when data is being "processed". This results in many non-sequential inserts. Later after processing, more data is inserted into this table by some of our in-house software. The fragmentation for a given table is at least 50%. In the case of the largest table, it is at 90%. We do not have a DBA. I am aware we desperately need a DB maintenance strategy. As far as my background, I'm a college student working part time at this company.
My question is this, is a clustered index the best way to go about this? Should another index be considered? Are there any good references for this type and similar ad-hoc DBA tasks?
The indexing strategy entirely depends on how you query the table and how much performance you need to get out of the respective queries.
A clustered index can force re-sorting rows physically (on disk) when out-of-sequence inserts are made (this is called "page split"). In a large table with no free space on the index pages, this can take some time.
If you are not absolutely required to have a clustered index spanning two fields, then don't. If it is more like a kind of a UNIQUE constraint, then by all means make it a UNIQUE constraint. No re-sorting is required for those.
Determine what the typical query against the table is, and place indexes accordingly. The more indexes you have, the slower data changes (INSERTs/UPDATEs/DELETEs) will go. Don't create too many indexes, e.g. on fields that are unlikely to be filtered/sorted on.
Create combined indexes only on fields that are filtered/sorted on together, typically.
Look hard at your queries - the ones that hit the table for data. Will the index serve? If you have an index on (ResultID, FieldName) in that order, but you are querying for the possible ResultID values for a given Fieldname, it is likely that the DBMS will ignore the index. By contrast, if you have an index on (FieldName, ResultID), it will probably use the index - certainly for simple value lookups (WHERE FieldName = 'abc'). In terms of uniqueness, either index works well; in terms of query optimization, there is (at least potentially) a huge difference.
Use EXPLAIN to see how your queries are being handled by your DBMS.
Clustered vs non-clustered indexing is usually a second-order optimization effect in the DBMS. If you have the index correct, there is a small difference between clustered and non-clustered index (with a bigger update penalty for a clustered index as compensation for slightly smaller select times). Make sure everything else is optimized before worrying about the second-order effects.
The clustered index is OK as far as I see. Regarding other indexes you will need to provide typical SQL queries that operate on this table. Just creating an index out of the blue is never a good idea.
You're talking about fragmentation and indexing, does it mean that you suspect that query execution slows down? Or do you simply want to shrink/defragment the database/index?
It is a good idea to have a task to defragment indexes from time to time during off-hours, though you have to consider that with frequent/random inserts it does not hurt to have some spare space in the table to prevent page splits (which do affect performance).
I am aware we desperately need a DB maintenance strategy.
+1 for identifying that need
As far as my background, I'm a college student working part time at this company
Keep studying, gain experience, but get an experienced consultant in in the meantime.
The table is populated by 24 workers running 4 threads each
I presume this is pretty mission critical during the working day, and downtime is bad news? If so don't clutz with it.
There is a clustered index on ResultID and Fieldname
Is ResultID the first column in the PK, as you indicate?
If so I'll bet that it is insufficiently selective and, depending on what the needs are of the queries, the order of the PK fields should be swapped (notwithstanding that this compound key looks to be a poor choice for the clustered PK)
What's the result of:
SELECT COUNT(*), COUNT(DISTINCT ResultID) FROM MyTable
If the first count is, say, 4 x as big as the second, or more, you will most likely be getting scans in preference to seeks, because of the low selectively of ResultsID, and some simple changes will give huge performance improvements.
Also, Fieldname is quite wide (50 chars) so any secondary indexes will have 50 + 4 bytes added to every index entry. Are the fields really CHAR rather than VARCHAR?
Personally I would consider increased the density of the leaf pages. At 90% you will only leave a few gaps - maybe one-per-page. But with a large table of 500 million rows the higher packing density may mean fewer levels in the tree, and thus fewer seeks for retrieval. Against that almost every insert, for a given page, will require a page split. This would favour inserts that are clustered, so may not be appropriate (given that your insert data is probably not clustered). Like many things, you'd need to make a test to establish what index key density works best. SQL Server has tools to help analyse how queries are being parsed, whether they are being cached, how many scans of the table they cause, which queries are "slow running", and so on.
Get a consultant in to take a look and give you some advice. This aint a question that answers here are going to give you a safe solution to implement.
You really REALLY need to have some carefully thought through maintenance policies for tables that have 500 millions rows and shed-loads of inserts daily. Sorry, but I have enormous frustration with companies that get into this state.
The table needs defragmenting (your options will become fewer if you don't have a clustered index, so keep that until you decide that there is a better candidate). "Online" defragmentation methods will have modest impact on performance, and can chug away - and can safely be aborted if they overrun time / CPU constraints [although that will most likely take some programming]. If you have a "quiet" slot then use it for table defragmentation and updating the statistics on indexes. Don't wait until the weekend to try to do all tables in one go - do as much/many as you can during any quiet time daily (during the night presumably).
Defragmenting the tables is likely to lead to a huge increased in Transaction log usage, so make sure that any TLogs are backed up frequently (we have a 10 minute TLog backup policy, which we increase to every minute during table defragging so that the defragging process doesn't become the definition of required Tlog space!)