In Relational Model, why we not keep all our data in a single table ? Why we need to create multiple tables ?
It depends on what your purpose is. For many analytic purposes, a single table is the simplest method.
However, relational databases are really design to keep data integrity. And one aspect of data integrity is that any given item of data is stored in only one place. For instance, a customer name is stored on the customer table, so it does not need to be repeated on the orders table.
This ensures that the customer name is always correct, because it is stored in one place.
In addition, repeating data through a single table often requires duplicating data -- and that would make the table way larger than needed and hence slow everything down.
I am not an expert, but i think that this would consume very much time and many resources, if there are a lot of data in the table. If we seperate them we can make operations a lot easier
Taken from https://www.bbc.co.uk/bitesize/guides/zvq634j/revision/1
A single flat-file table is useful for recording a limited amount of
data. But a large flat-file database can be inefficient as it takes up
more space and memory than a relational database. It also requires new
data to be added every time you enter a new record, whereas a
relational database does not. Finally, data redundancy – where data is
partially duplicated across records – can occur in flat-file tables,
and can more easily be avoided in relational databases.
Therefore, if you have a large set of data about many different
entities, it is more efficient to create separate tables and connect
them with relationships.
Related
I'm curious if there are any trade offs between creating a child table to hold a set of data compared to just placing all the data in the main table in the first place?
My scenario is that I have data that handles various metrics. Such as LastUpdated, and AmtOfXXX. I'm curious if it would be better to place all this data in a Table (specifically for metrics) and reference it by foreign key, or place all these fields directly in the main table and forego any foreign keys? Are there trade-offs? Performance considerations?
I'm referring to Relational Database Management Systems such as SQL Server and specifically I'll be using Entity Framework Core with MS SQL Server.
Your question appears to be more about the considerations between the two approach rather than asking which is specifically better. The latter is more an opinion. This addresses the former.
The major advantage to having a separate table that is 1-1 is to isolate the metrics from other information about the entities. There is a name for this type of data model, vertical partitioning (or at least that's what it was called when I first learned about it).
This has certain benefits:
The width of the data rows is smaller. So queries that only need the "real" data (or only the metrics) are faster.
The metrics are isolated. So adding new metrics does not require rewriting the "real" data.
A query such as select * on the "real" data only returns the real data.
Queries that modify only the metrics do not lock the "real" data.
There might also be an edge case if you have lots of columns and they fit into two tables but not into one.
Of course, there is overhead:
You need a JOIN to connect the two tables. (Although with the same primary key, the join will be quite fast).
Queries that modify both the "real" data and the metrics are more complicating, having to lock both tables.
I want to move multiple SQLite files to PostgreSQL.
Data contained in these files are monthly time-series (one month in a single *.sqlite file). Each has about 300,000 rows. There are more than 20 of these files.
My dilemma is how to organize the data in the new database:
a) Keep it in multiple tables
or
b) Merge it to one huge table with new column describing the time period (e.g. 04.2016, 05.2016, ...)
The database will be used only to pull data out of it (with the exception of adding data for new month).
My concern is that selecting data from multiple tables (join) would not perform very well and the queries can get quite complicated.
Which structure should I go for - one huge table or multiple smaller tables?
Think I would definitely go for one table - just make sure you use sensible indexes.
If you have the space and the resource 1 table, as other users have appropriately pointed out databases can handle millions of rows no problem.....Well depends on the data that is in them. The row size can make a big difference... Such as storing VARCHAR(MAX), VARBINARY(MAX) and several per row......
there is no doubt writing queries, ETL (extract transform load) is significantly easier on a single table! And maintenance of that is easier too from a archival perspective.
But if you never access the data and you need the performance in the primary table some sort of archive might make since.
There are some BI related reasons to maintain multiple tables but it doesn't sound like that is your issue here.
There is no perfect answer and will depend on your situation.
PostgreSQL is easily able to handle millions of rows in a table.
Go for option b) but..
with new column describing the time period (e.g. 04.2016, 05/2016, ...)
Please don't. Querying the different periods will become a pain, an unnecessary one. Just put the date in one column, put a index on the column and you can, probably, execute fast queries on it.
My concern is that selecting data from multiple tables (join) would not perform very well and the queries can get quite complicated.
Complicated for you to write or for the database to execute? An Example would be nice for us to get an image of your actual requirements.
Is there any performance benefit to splitting a large table with roughly 100 columns into 2 separate tables? This would be in terms of inserting, deleting and selecting tasks? I'm using SQL Server 2008.
If one of the fields is a CLOB or BLOB and you anticipate it holding a huge amount of data and you won't need that field very often and the result set will transmitted over a long pipe (like server to a web-based client), then I think putting that field in a separate table would be appropriate.
But just returning 100 regular fields probably won't tax your system so much as to justify a separate table and a join.
The only benefit you might see is if a number of columns are only occasionally populated. In which case putting those into their own table and only adding a row when there is data might make sense in terms of overall row overhead and, depending on the number of rows, overall page count for the table(s). That said, this is one of the reasons they introduced sparse columns in SQL Server 2008.
For the maintenance and other overhead of managing two tables instead of one (especially given that people can act on individual tables if they choose), it's unlikely it would be worth it.
Can you describe what type of entity needs to have over 100 columns? Perhaps the data model is just wrong in the first place.
I would say no as it would take more execution time to join the 2 tables whenever you wanted to do something.
I depends if you use these fields in the same time in your application.
These kind of performance improvements are really bad : you make your source code impossible to understand. If you have performance trouble with this table, add something (like a table containing the 15 fields you'll use in a request that'll updated via trigger), don't modify your clean solution.
If you don't have performance problem, don't do anything, you'll see later !
I'm setting up a table that might have upwards of 70 columns. I'm now thinking about splitting it up as some of the data in the columns won't be needed every time the table is accessed. Then again, if I do this I'm left with having to use joins.
At what point, if any, is it considered too many columns?
It's considered too many once it's above the maximum limit supported by the database.
The fact that you don't need every column to be returned by every query is perfectly normal; that's why SELECT statement lets you explicitly name the columns you need.
As a general rule, your table structure should reflect your domain model; if you really do have 70 (100, what have you) attributes that belong to the same entity there's no reason to separate them into multiple tables.
There are some benefits to splitting up the table into several with fewer columns, which is also called Vertical Partitioning. Here are a few:
If you have tables with many rows, modifying the indexes can take a very long time, as MySQL needs to rebuild all of the indexes in the table. Having the indexes split over several table could make that faster.
Depending on your queries and column types, MySQL could be writing temporary tables (used in more complex select queries) to disk. This is bad, as disk i/o can be a big bottle-neck. This occurs if you have binary data (text or blob) in the query.
Wider table can lead to slower query performance.
Don't prematurely optimize, but in some cases, you can get improvements from narrower tables.
It is too many when it violates the rules of normalization. It is pretty hard to get that many columns if you are normalizing your database. Design your database to model the problem, not around any artificial rules or ideas about optimizing for a specific db platform.
Apply the following rules to the wide table and you will likely have far fewer columns in a single table.
No repeating elements or groups of elements
No partial dependencies on a concatenated key
No dependencies on non-key attributes
Here is a link to help you along.
That's not a problem unless all attributes belong to the same entity and do not depend on each other.
To make life easier you can have one text column with JSON array stored in it. Obviously, if you don't have a problem with getting all the attributes every time. Although this would entirely defeat the purpose of storing it in an RDBMS and would greatly complicate every database transaction. So its not recommended approach to be followed throughout the database.
Having too many columns in the same table can cause huge problems in the replication as well. You should know that the changes that happen in the master will replicate to the slave.. for example, if you update one field in the table, the whole row will be w
I know it's probably not the right way to structure a database but does the database perform faster if the data is put in one huge table instead of breaking it up logically in other tables?
I want to design and create the database properly using keys to create relational integrity across tables but when quering, is JOIN'ing slower than reading the required data from one table? I want to make the database queries as fast as possible.
So many other facets affect the answer to your question. What is the size of the table? width? how many rows? What is usage pattern? Are there different usage patterns for different subsets of the columns in the table? (i.e., are two columns hit 1000 times per second, and the other 50 columns only hit once or twice a day? ) this scenario would be a prime candidate to split (partition) the table vertically (two columns in one table, the rest on another)
In general, normalize the schema to the maximum degree possible, then run performance testing with typical or predicted loads and usage patterns, and denormalize and partition to the point where the performance becomes acceptable, and no more...
It depends on the dbms flavor and your actual data, of course. But generally more smaller (narrower) tables are faster than fewer larger (wider) tables.
Access is a little slower when joins must be performed. How much slower depends greatly on the features offered by your particular DBMS, and how the physical database design exploits those features, and on the most frequent access patterns. There are a few access patterns where storing a lot of data in one row wastes time, because the entire row is retrieved, but only a little of the row is used. It depends.
When data is stored in a single table and the normalization rules are deviated from, update is typically slower. How important speed of of update is versus speed of query is dependant on the particular way you use this database.
In general, a lot of newbie database designers tend to put more weight on speed issues than those issues deserve. If your data model is inflexible and incomprehensible, but you gain a 10% speed improvement, you have probably done more harm than good.
Are you building a "read-only" database like a data warehouse? If so, storing data "pre-joined" may make sense. For everyday OLTP databases you need to take into account the performance and ease of inserts, updates and deletes as well. Also, what about queries that only want the data that would have been in one or two of the smaller tables? Now they have to grind through a big fat table full of stuff they don't care about.
It's worth remembering that joining tables is bread-and-butter stuff to a decent DBMS - they are very good at it.
It is often true that querying a single table is faster than querying multiple joined tables. But a normalized design allows you to query the data in multiple ways, with adequate performance across many types of queries.
If you denormalize the tables, you may improve performance of one specific query, while sacrificing performance of other queries against that data. And of course you'll have to manage referential integrity and redundancy manually.
What you're asking about is denormalization - it can speed up reads if done in the right way, and if you are able to ensure that you're not introducing anomalies into your database because of it.
Remember also that there is a hard limit to the amount of data that can be stored in one record. (not knowing which database you have, I can't say what it is.) Too many columns and you will hit that limit. Also if you are having columns like phone1, phone2, phone3 then you need to normalize. If you would need to add a column if the number of items to be inserted about a record changes (if you statred needing 4 instead of 3 phone numbers for instance), you need to normalize instead.
What's true for optimising SELECTS is often not so great at optimising INSERTS, UPDATES and DELETES, and thus it is with this approach. Breaking out the data into properly normalised tables reduces the overhead of changing the data.
While it's tru that in a data warehouse or decision suport system we'd often store pre-joined data (as Tony says), it usually only happens in the context of a precomputed summary (eg. a materialized view) and not for data at the atomic level of granularity. The reason for this is that pushing repeated longer character strings (eg. "Supplier Name") into a dimension table reduces total required storage space and number of physical reads required to retrieve the data. The joins are usually equijoins, and these are performed at almost no cost for large data sets.