I want to order my table Jogadores by Total value which I can achieve by writing this:
ALTER TABLE `Jogadores` ORDER BY `Total` DESC ;
My question is how do I use this as a trigger every time I edit a Total value or insert a new row?
I'm using phpMyAdmin
Thanks
I'm not sure I understand what you are trying to accomplish. I am going to assume you want your table sorted in that fashion for when you retrieve it. If that is true, you can sort it upon retrieval rather than altering the table.
SELECT *
FROM Jogadores
ORDER BY Total DESC;
Why? That's not usually how SQL databases work
What if some totals are the same? What is the second way to sort them?
It's also usually not a good idea to store a total in your operational db. Is this a data warehouse?
It would probably be easier to add an index on total and create a view to order by that column.
After reading the new comments, what you want is not doable. See this answer.
Original content:
As others have said, the physical order of the rows in the table doesn't matter since you can always sort when querying. If you're thinking of clustered indexes, then all you need to do is define your primary key properly.
The InnoDB term for a primary key index. InnoDB table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. For best performance, choose the primary key columns carefully based on the most performance-critical queries. Because modifying the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never updated.
So a clustered index would achieve what you want, but it is probably not what you actually need. Just so you know, the clustered index speedups are practically zero if you're dealing with less than a million or so rows (rough estimate).
Related
I'm looking for a partitioning solution. And, obviously, I'm interested in the performance side of it. Means, not to make it too bad while benefiting maintenance time.
My question is related to one of the tables that I have. And it looks similar to
Id bigint IDENTITY PRIMARY KEY CLUSTERED,
DatetimeFiled,
/*
a lot of other fields
*/
According to the data structure and usage, it's suitable to split the table into partitions by the DatetimeFiled (classic), because I have filters by date on this table.
But I do have filters by Id as well. Moreover, I have JOINs that use the Id field as a predicate, which now benefits from its uniqueness (https://www.brentozar.com/archive/2015/08/performance-benefits-of-unique-indexes/).
So, I decided to use Id ,DatetimeFiled as UNIQUE CLUSTERED INDEX.
But I doubt will it still benefit JOINs by Id field?
And is it ok to use that kind of field order, because I saw that partitioned field is often in the first place?
Using a trailing clustered index column as the partition column is a common and useful approach. You can find rows by Id by seeking the Clustered Index in each partition, and you can find rows by DatetimeFiled through partition elimination.
In our data warehouse (SQL Server 2005), we attempt to insert/update records in order of the primary key. In other words, we pull from the source table and issue an ORDER BY primary key in DW. This is a standard practice to keep the data reads/writes in logical order on the hard drive and improve performance. (If this is not accurate, please let me know).
When issuing an ORDER BY on a very large source table, this really kills performance. Is there another way to get the same result? I am thinking some combination of index rebuilds and computing stats?
Hope that makes sense! I'm not a DBA! Thanks.
If you want to force a specific ordering, you must have an order by clause. No ifs, ands, or buts. A clustered index on the column you want to sort by would probably make the select run faster, but building that index may end up taking just as long as your original query.
I would look into alternative methods of re-ordering your target table (as suggested in Johnny Bones's reply).
If that table does not have an index, then the pages aren't going to be stored with any particular order on the disk. In the case of a large table without any index, a SELECT.....ORDER BY from said table will have a performance issue.
It sounds like you need an index on your primary key.
If the table you are pulling from has a clustered index on the PK (which it should), then bringing back records in the order of the PK should have zero performance impact. Any performance issues you are having is probably due to the sheer number of records you are returning (not to the ORDER BY).
I don't think I understand what you're trying to do, exactly. But bringing back the records in that order should not be a problem.
I don't know if this is "standard practice", but I worked in a warehouse where our typical table had close to a billion records. We would always drop the indexes, insert the new data, and then rebuild the indexes. Someone, at some point, determined this was the most efficient way for us to do it. I'm sure someone here can chime in about page size and physical attributes (which is probably more than you need to know, since you say you're not a DBA), but the short answer is to do it that way.
If you decide to go that route, always remember to drop the non-clustered indexes first, and then drop the clustered. When you rebuild them, rebuild them in reverse order (clustered first and then non-clustered).
Which type of index should be used on the table? It is initially inserted (one a month) into a empty table. I then place a non clustered composite index on two of the columns. Wondering if merging the two fields into one would increase performance when searching. Or does it not matter? Should I be working with an identity column that has a primary key clustered index?
You should index the field(s) most likely to be used in the where clause as people query the table. Don't worry about the primary key - it already has an index.
If you can define a unique primary key that can be used when querying the table, this will be used as the clustered index and will be the fastest for selects.
If your select query has to use the two fields you mentioned, keep them separate. Performance will not be impacted and the schema is not spoiled.
"A clustered index is particularly efficient on columns that are often searched for ranges of values. After the row with the first value is found using the clustered index, rows with subsequent indexed values are guaranteed to be physically adjacent."
With this in mind you probably won't see much benefit from haveing a clustered index on your primary key (ID) unless it have business meaning for your aplication. If you have a Date value that you are commonly querying, then it may make more sense to add a clustered index to that
select * from table where created > '2013-01-01' and created < '2013-02-01'
I have seen datawarehouses use a concatenated key approach. Whether this works for you depends on your queries. Obviously querying a single field value will be faster than multiple fields, particularly when there is one less lookup in the B-tree index.
Alternatively, if you have 200 million rows in a table you could look at breaking the data out into multiple tables if it makes sense to do so.
You're saying that you're loading all this data every month so I have to assume that all the data is relevant. If there was data in your table that is considered "old" and not relevant to searches, then you could move data out into a archive table (using the same schema) so your queries only run against "current" data.
Otherwise, you can look at a sharding approach as used by NoSQL like MongoDB. If MongoDB is not an option, you could achieve the same shard key like logic in your application. I doubt that your database SQL drivers will support sharding natively.
Are there any techniques which can be used to simulate multiple clustered indices on a table in Sybase 12.5 ? thanks
I dont think, you can simulate multiple clustered indices. Because when you have one clustered index created on a table, the data gets rearranged according to the data in the clustered index column. Logically you cannot arrange the data in another order in a table according to another column. All that you can do is create non clustered index for other columns
Other thing you can do is combine two or more columns and create a clustered index.
The only close approximation I can think of for this would be to create non-clustered indexes that include all columns from the table. In that way, the non-clustered index would contain all of the data.
However, to achieve that, the entire table would have to fit within any constraints imposed on non-clustered indexes. (E.g. for SQL Server, there's a limit on some column datatypes, and the whole size in bytes - probably similar restrictions apply in any product).
Best way is to create as many tables as you want and try different clustered indexes,which you want to simulate, on these tables. Then maybe run queries against these tables to check which one is performing better ,if your main motive is just to check which column would make a better clustered index. But i would advice that you should run full workload or all the queries which you will be executing against this table so that you will be in better position to see which combinations will be best for you.
Tabe1 has around 10 Lack records (1 Million) and does not contain any primary key. Retrieving the data by using SELECT command ( With a specific WHERE condition) is taking large amount of time. Can we reduce the time of retrieval by adding a primary key to the table or do we need to follow any other ways to do the same. Kindly help me.
A primary key does not have a direct affect on performance. But indirectly, it does. This is because when you add a primary key to a table, SQL Server creates a unique index (clustered by default) that is used to enforce entity integrity. But you can create your own unique indexes on a table. So, strictly speaking, a primary index does not affect performance, but the index used by the primary key does.
WHEN SHOULD PRIMARY KEY BE USED?
Primary key is needed for referring to a specific record.
To make your SELECTs run fast you should consider adding an index on an appropriate columns you're using in your WHERE.
E.g. to speed-up SELECT * FROM "Customers" WHERE "State" = 'CA' one should create an index on State column.
Primarykey will not help if you don't have Primarykey in where cause.
If you would like to make you quesry faster, you can create non-cluster index on columns in where cause. You may want include columns on top of your index(it depend on your select cause)
The SQL optimizer will seek on your indexs that will make your query faster.
(but you should think about when data adding in your table. Insert operation might takes time if you create index on many columns.)
It depends on the SELECT statement, and the size of each row in the table, the number of rows in the table, and whether you are retrieving all the data in each row or only a small subset of the data (and if a subset, whether the data columns that are needed are all present in a single index), and on whether the rows must be sorted.
If all the columns of all the rows in the table must be returned, then you can't speed things up by adding an index. If, on the other hand, you are only trying to retrieve a tiny fraction of the rows, then providing appropriate indexes on the columns involved in the filter conditions will greatly improve the performance of the query. If you are selecting all, or most, of the rows but only selecting a few of the columns, then if all those columns are present in a single index and there are no conditions on columns not in the index, an index can help.
Without a lot more information, it is hard to be more specific. There are whole books written on the subject, including:
Relational Database Index Design and the Optimizers
One way you can do it is to create indexes on your table. It's always better to create a primary key, which creates a unique index that by default will reduce the retrieval time .........
The optimizer chooses an index scan if the index columns are referenced in the SELECT statement and if the optimizer estimates that an index scan will be faster than a table scan. Index files generally are smaller and require less time to read than an entire table, particularly as tables grow larger. In addition, the entire index may not need to be scanned. The predicates that are applied to the index reduce the number of rows to be read from the data pages.
Read more: Advantages of using indexes in database?