I have a complex process that interacts with multiple systems.
Each of these systems may produce error messages that I would like to store in a table of my Oracle database (note that I have statuses but the nature of the process is such that the errors may not always be predefined).
We are talking about hundred thousands of transactions each day where 1% may result in various errors.
1) Wanted to know what is a reasonable/acceptable length for the database field and how big of a message should I be storing?
2) Memory wise, does it really matter how large the field is defined in the database?
"Reasonable" and "acceptable" depends on the application. Assuming that you want to define the database column as a VARCHAR2 rather than a CLOB, and assuming that you aren't using 12.1 or later, you can declare the column to hold up to 4000 bytes. Is that enough for whatever error messages you need to support? Is there a lower limit on the length of an error message that you can establish? If you're producing error messages that are designed to be shown to a user, you're probably going to be generating shorter messages. If you're producing and storing stack traces, you may need to declare the column as a CLOB because 4000 bytes may not be sufficient.
What sort of memory are we talking about? On disk, a VARCHAR2 will only allocate the space that is actually required to store the data. When the block is read into the buffer cache, it will also only use the space required to store the data. If you start allocating local variables in PL/SQL, depending on the size of the field, Oracle may allocate more space than is required to store the particular data for that local variable in order to try to avoid the cost of growing and shrinking the allocation when you modify the string. If you return the data to a client application (including a middle tier application server), that client may allocate a buffer in memory based on the maximum size of the column rather than based on the actual size of the data.
Related
I've read up on this on MSDN forums and here and I'm still not clear. I think this is correct: Varchar(max) will be stored as a text datatype, so that has drawbacks. So lets say your field will reliably be under 8000 characters. Like a BusinessName field in my database table. In reality, a business name will probably always be under (pulling a number outta my hat) 500 characters. It seems like plenty of varchar fields that I run across fall well under the 8k character count.
So should I make that field a varchar(500) instead of varchar(8000)? From what I understand of SQL there's no difference between those two. So, to make life easy, I'd want to define all my varchar fields as varchar(8000). Does that have any drawbacks?
Related: Size of varchar columns (I didn't feel like this one answered my question).
One example where this can make a difference is that it can prevent a performance optimization that avoids adding row versioning information to tables with after triggers.
This is covered by Paul White here
The actual size of the data stored is immaterial – it is the potential
size that matters.
Similarly if using memory optimised tables since 2016 it has been possible to use LOB columns or combinations of column widths that could potentially exceed the inrow limit but with a penalty.
(Max) columns are always stored off-row. For other columns, if the data row size in the table definition can exceed 8,060 bytes, SQL Server pushes largest variable-length column(s) off-row. Again, it does not depend on amount of the data you store there.
This can have a large negative effect on memory consumption and performance
Another case where over declaring column widths can make a big difference is if the table will ever be processed using SSIS. The memory allocated for variable length (non BLOB) columns is fixed for each row in an execution tree and is per the columns' declared maximum length which can lead to inefficient usage of memory buffers (example). Whilst the SSIS package developer can declare a smaller column size than the source this analysis is best done up front and enforced there.
Back in the SQL Server engine itself a similar case is that when calculating the memory grant to allocate for SORT operations SQL Server assumes that varchar(x) columns will on average consume x/2 bytes.
If most of your varchar columns are fuller than that this can lead to the sort operations spilling to tempdb.
In your case if your varchar columns are declared as 8000 bytes but actually have contents much less than that your query will be allocated memory that it doesn't require which is obviously inefficient and can lead to waits for memory grants.
This is covered in Part 2 of SQL Workshops Webcast 1 downloadable from here or see below.
use tempdb;
CREATE TABLE T(
id INT IDENTITY(1,1) PRIMARY KEY,
number int,
name8000 VARCHAR(8000),
name500 VARCHAR(500))
INSERT INTO T
(number,name8000,name500)
SELECT number, name, name /*<--Same contents in both cols*/
FROM master..spt_values
SELECT id,name500
FROM T
ORDER BY number
SELECT id,name8000
FROM T
ORDER BY number
From a processing standpoint, it will not make a difference to use varchar(8000) vs varchar(500). It's more of a "good practice" kind of thing to define a maximum length that a field should hold and make your varchar that length. It's something that can be used to assist with data validation. For instance, making a state abbreviation be 2 characters or a postal/zip code as 5 or 9 characters. This used to be a more important distinction for when your data interacted with other systems or user interfaces where field length was critical (e.g. a mainframe flat file dataset), but nowadays I think it's more habit than anything else.
There are some disadvantages to large columns that are a bit less obvious and might catch you a little later:
All columns you use in an INDEX - must not exceed 900 bytes
All the columns in an ORDER BY clause may not exceed 8060 bytes. This is a bit difficult to grasp since this only applies to some columns. See SQL 2008 R2 Row size limit exceeded for details)
If the total row size exceeds 8060 bytes, you get a "page spill" for that row. This might affect performance (A page is an allocation unit in SQLServer and is fixed at 8000 bytes+some overhead. Exceeding this will not be severe, but it's noticable and you should try to avoid it if you easily can)
Many other internal datastructures, buffers and last-not-least your own varaibles and table-variables all need to mirror these sizes. With excessive sizes, excessive memory allocation can affect performance
As a general rule, try to be conservative with the column width. If it becomes a problem, you can easily expand it to fit the needs. If you notice memory issues later, shrinking a wide column later may become impossible without losing data and you won't know where to begin.
In your example of the business names, think about where you get to display them. Is there really space for 500 characters?? If not, there is little point in storing them as such. http://en.wikipedia.org/wiki/List_of_companies_of_the_United_States lists some company names and the max is about 50 characters. So I'd use 100 for the column max. Maybe more like 80.
Apart from best practices (BBlake's answer)
You get warnings about maximum row size (8060) bytes and index width (900 bytes) with DDL
DML will die if you exceed these limits
ANSI PADDING ON is the default so you could end up storing a wholeload of whitespace
Ideally you'd want to go smaller than that, down to a reasonably sized length (500 isn't reasonably sized) and make sure the client validation catches when the data is going to be too large and send a useful error.
While the varchar isn't actually going to reserve space in the database for the unused space, I recall versions of SQL Server having a snit about database rows being wider than some number of bytes (do not recall the exact count) and actually throwing out whatever data didn't fit. A certain number of those bytes were reserved for things internal to SQL Server.
I know basic SQL/RDBMS and not the kind of detailed information a highly experienced DBA would know.
I need to know how much memory is consumed by a variable such as int, bigint, date time. I also need to know how much memory is consumed by a Varchar(50) column in two cases -
1] The Column is filled with strings of size 50
2] Column has all Null
The purpose behind this is to make estimates for ETL/data transfer.
I also want to know how to store SQL server result into a cache on disk and then retrieve the data from that cache, chunk by chunk (doing this due to memory related concerns). But, I'll make that another question.
In addition to the documentation linked in the comment, note that varchar storage depends on what data is actually entered.
From http://technet.microsoft.com/en-us/library/ms176089(v=sql.100).aspx:
The storage size is the actual length of data entered + 2 bytes.
Suppose I have a table with a column name varchar(20), and I store a row with name = "abcdef".
INSERT INTO tab(id, name) values(12, 'abcdef');
How is the memory allocation for name done in this case?
There are two ways I can think of:
a)
20 bytes is allocated but only 6 used. In this case varchar2 does not have any significant advantage over char, in terms of memory allocation.
b)
Only 6 bytes is allocated. If this is the case, and I addded a couple of more rows after this one,
INSERT INTO tab(id, name) values(13, 'yyyy');
INSERT INTO tab(id, name) values(14, 'zzzz');
and then I do a UPDATE,
UPDATE tab SET name = 'abcdefghijkl' WHERE id = 12;
Where does the DBMS get the extra 6 bytes needed from? There can be a case that the next 6 bytes are not free (if only 6 were allocated initially, next bytes might have been allotted for something else).
Is there any other way than shifting the row out to a new place? Even shifting would be a problem in case of index organized tables (it might be okay for heap organized tables).
There may be variations depending on the rdbms you are using, but generally:
Only the actual data that you store in a varchar field is allocated. The size is only a maximum allowed, it's not how much is allocated.
I think that goes for char fields also, on some systems. Variable size data types are handled efficiently enough that there is no longer any gain in allocating the maximum.
If you update a record so that it needs more space, the record inside the same allocation block are moved down, and if the records no longer fit in the block, another block is allocated and the records are distributed between the blocks. That means that records are continous inside the allocation blocks, but the blocks doesn't have to be continous on the disk.
It certainly doesn't allocate more space then needed, this would defeat the point of using the variable length type.
In the case you mention I would think that the rows below would have to be moved down on the page, perhaps this is optimized somehow. I don't really know the exact details, perhaps someone else can comment further.
This is probably heavily database dependent.
A couple of points though: MVCC observing databases don't actually update data on disk or in memory cache. They insert a new row with the updated data and mark the old row as deleted from a certain transaction on. After a while the deleted row is not visible to any transactions and it's reclaimed.
For the space storage issue, it's usually in the form of 1-4 bytes of header + data (+ padding)
In the case of chars, the data is padded to reach the sufficient length. In the case of varchar or text, the header stores the length of the data that is following.
Edit For some reason I thought this was tagged Microsoft SQL Server. I think the answer is still relevant though
That's why the official recommendation is
Use char when the sizes of the column data entries are consistent.
Use varchar when the sizes of the column data entries vary considerably.
Use varchar(max) when the sizes of the column data entries vary
considerably, and the size might
exceed 8,000 bytes.
It's a trade off you need to consider when designing your table structure. Probably you would need to consider the frequency of updates vs reads in this calculation too
Worth noting that for char a NULL value still uses all the storage space. There is an addin for Management Studio called SQL Internals Viewer that allows you to see easily how your rows are stored.
Given the VARCHAR2 in the question title, I assume your question is focused around Oracle. In Oracle, you can reserve space for row expansion within a data block with the use of the PCTFREE clause. That can help mitigate the effects of updates making rows longer.
However, if Oracle doesn't have enough free space within the block to write the row back, what it does it is called row migration; it leaves the original address on disk alone (so it doesn't necessarily need to update indexes), but instead of storing the data in the original location, it stores a pointer to that row's new address.
This can cause performance problems in cases where a table is heavily accessed by indexes if a significant number of the rows have migrated, as it adds additional I/O to satisfy queries.
I've read up on this on MSDN forums and here and I'm still not clear. I think this is correct: Varchar(max) will be stored as a text datatype, so that has drawbacks. So lets say your field will reliably be under 8000 characters. Like a BusinessName field in my database table. In reality, a business name will probably always be under (pulling a number outta my hat) 500 characters. It seems like plenty of varchar fields that I run across fall well under the 8k character count.
So should I make that field a varchar(500) instead of varchar(8000)? From what I understand of SQL there's no difference between those two. So, to make life easy, I'd want to define all my varchar fields as varchar(8000). Does that have any drawbacks?
Related: Size of varchar columns (I didn't feel like this one answered my question).
One example where this can make a difference is that it can prevent a performance optimization that avoids adding row versioning information to tables with after triggers.
This is covered by Paul White here
The actual size of the data stored is immaterial – it is the potential
size that matters.
Similarly if using memory optimised tables since 2016 it has been possible to use LOB columns or combinations of column widths that could potentially exceed the inrow limit but with a penalty.
(Max) columns are always stored off-row. For other columns, if the data row size in the table definition can exceed 8,060 bytes, SQL Server pushes largest variable-length column(s) off-row. Again, it does not depend on amount of the data you store there.
This can have a large negative effect on memory consumption and performance
Another case where over declaring column widths can make a big difference is if the table will ever be processed using SSIS. The memory allocated for variable length (non BLOB) columns is fixed for each row in an execution tree and is per the columns' declared maximum length which can lead to inefficient usage of memory buffers (example). Whilst the SSIS package developer can declare a smaller column size than the source this analysis is best done up front and enforced there.
Back in the SQL Server engine itself a similar case is that when calculating the memory grant to allocate for SORT operations SQL Server assumes that varchar(x) columns will on average consume x/2 bytes.
If most of your varchar columns are fuller than that this can lead to the sort operations spilling to tempdb.
In your case if your varchar columns are declared as 8000 bytes but actually have contents much less than that your query will be allocated memory that it doesn't require which is obviously inefficient and can lead to waits for memory grants.
This is covered in Part 2 of SQL Workshops Webcast 1 downloadable from here or see below.
use tempdb;
CREATE TABLE T(
id INT IDENTITY(1,1) PRIMARY KEY,
number int,
name8000 VARCHAR(8000),
name500 VARCHAR(500))
INSERT INTO T
(number,name8000,name500)
SELECT number, name, name /*<--Same contents in both cols*/
FROM master..spt_values
SELECT id,name500
FROM T
ORDER BY number
SELECT id,name8000
FROM T
ORDER BY number
From a processing standpoint, it will not make a difference to use varchar(8000) vs varchar(500). It's more of a "good practice" kind of thing to define a maximum length that a field should hold and make your varchar that length. It's something that can be used to assist with data validation. For instance, making a state abbreviation be 2 characters or a postal/zip code as 5 or 9 characters. This used to be a more important distinction for when your data interacted with other systems or user interfaces where field length was critical (e.g. a mainframe flat file dataset), but nowadays I think it's more habit than anything else.
There are some disadvantages to large columns that are a bit less obvious and might catch you a little later:
All columns you use in an INDEX - must not exceed 900 bytes
All the columns in an ORDER BY clause may not exceed 8060 bytes. This is a bit difficult to grasp since this only applies to some columns. See SQL 2008 R2 Row size limit exceeded for details)
If the total row size exceeds 8060 bytes, you get a "page spill" for that row. This might affect performance (A page is an allocation unit in SQLServer and is fixed at 8000 bytes+some overhead. Exceeding this will not be severe, but it's noticable and you should try to avoid it if you easily can)
Many other internal datastructures, buffers and last-not-least your own varaibles and table-variables all need to mirror these sizes. With excessive sizes, excessive memory allocation can affect performance
As a general rule, try to be conservative with the column width. If it becomes a problem, you can easily expand it to fit the needs. If you notice memory issues later, shrinking a wide column later may become impossible without losing data and you won't know where to begin.
In your example of the business names, think about where you get to display them. Is there really space for 500 characters?? If not, there is little point in storing them as such. http://en.wikipedia.org/wiki/List_of_companies_of_the_United_States lists some company names and the max is about 50 characters. So I'd use 100 for the column max. Maybe more like 80.
Apart from best practices (BBlake's answer)
You get warnings about maximum row size (8060) bytes and index width (900 bytes) with DDL
DML will die if you exceed these limits
ANSI PADDING ON is the default so you could end up storing a wholeload of whitespace
Ideally you'd want to go smaller than that, down to a reasonably sized length (500 isn't reasonably sized) and make sure the client validation catches when the data is going to be too large and send a useful error.
While the varchar isn't actually going to reserve space in the database for the unused space, I recall versions of SQL Server having a snit about database rows being wider than some number of bytes (do not recall the exact count) and actually throwing out whatever data didn't fit. A certain number of those bytes were reserved for things internal to SQL Server.
What's the disadvantage of choosing a large value for max when creating a varchar or varbinary column?
I'm using MS SQL but I assume this would be relevant to other dbs as well.
Thanks
That depends on whether it is ever reasonable to store a large amount of data in the particular column.
If you declare a column that would never properly store much data (i.e. an employee first name as a VARCHAR(1000)), you end up with a variety of problems
Many if not most client APIs (i.e. ODBC drivers, JDBC drivers, etc) allocate memory buffers on the client that are large enough to store the maximum size of a particular column. So even though the database only has to store the actual data, you may substantially increase the amount of memory the client application uses.
You lose the ability to drive data validation rules (or impart information about the data) from the table definition. If the database allows 1000 character first names, every application that interacts with the database will probably end up having its own rules for how large an employee name can be. If this is not mitigated by putting a stored procedure layer between all applications and the tables, this generally leads to various applications having various rules.
Murphy's Law states that if you allow 1000 characters, someone will eventually store 1000 characters in the column, or at least a value large enough to cause errors in one or more application (i.e. no one checked to see whether every application's employee name field could display 1000 characters).
Depends on the RDBMS. IIRC, MySql allocates a 2 byte overhead for varchars > 255 characters (to track the varchar length). MSSQL <= 2000 would allow you to allocate a row size > 8060 bytes, but would fail if you tried to INSERT or UPDATE a row that actually exceeded 8060 bytes. SQL 2005[1] allows the insert, but will allocate a new page for the overflow and leave a pointer behind. This, obviously, impacts performance.
[1] varchar(max) is somewhat of a special case, but will also allocate an overflow page if the length of the field is > 8000 or the row > 8060. This is with MSSQL defaults, and behavior can change with the large types in data row option.
You could be adding a risk of breaking your application if a large data got in somehow (like from an external interface) and your app isn't designed to handle it.
As a good design, you should always limit the size of the fields to a realistic value.