I am getting below error. How to fix it? Why am I getting this error?
Message operation failed. The index entry of length 1526 bytes for the index ix_Emp_no_1 exceeds the maximum length of 900 bytes.
Maximum Bytes per index key
900 bytes for a clustered index.
1,700 for a nonclustered index
Result and meaning:
When you have a nchar(450) because Unicode characters occupy 2 bytes it will equal 900 bytes.
you can ignore Unicode and use char(900) instead.
for variable strings types like varchar and nvarchar it seems to be working in the same manner.
if you have composite key containing multiple columns the sum of them have to have lower bytes than this limitations.
Note : sometimes the SSMS designer does not let you create index larger than 900 bytes (till 1700 bytes) but it is officially supported. you just need to generate the script and run it yourself.
source : https://learn.microsoft.com/en-us/sql/sql-server/maximum-capacity-specifications-for-sql-server?view=sql-server-2017
I have never seen this, but I found this article that may help you deal with your situation.
https://blogs.msdn.microsoft.com/bartd/2011/01/06/living-with-sqls-900-byte-index-key-length-limit/
It sounds like you have a key that is over 900 bytes, a large text field maybe. So you need to find a different way to identify that row. Try to index a different column and include the problem column.
Related
I'm having trouble understanding how to define a column for my text that has the right size for my max. number of characters. In Oracle I can create a VARCHAR2(10 CHAR) which will be big enough for 10 characters. The size depends on the encoding used in the database. But how do I do that in SQL Server? Do I use varchar(10)? nvarchar(10)? I want to be able to store all kinds of characters (even chinese).
If you want Chinese characters, you need to use nvarchar(n) and specify a length of n that makes sense.
Those are characters you're defining, and the space you need is twice that number (since any Unicode character in SQL Server always uses 2 bytes).
Max. size is nvarchar(4000) - or if you really need more, use nvarchar(max) (for up to 1 billion characters).
I would recommend NOT to just use nvarchar(max) for everything, out of lazyness about considering what size you really need! Since it's a really large column, you won't be able to index it for one.
If you use nvarchar(max) this will allow for any number of characters for all character sets. The system will optimise storage.
Limitations on row size are addressed here. See answer from #marc_s for limitations on the use of max.
I have some data which I will be putting in the database. Say I make a field like "coupondetail text(10000)" which will store the coupon detail, now consider that not all coupondetail will be 10,000 chars long. I m curious to know how much space will the column take in the database when the coupondetail text is lesser than 10,000 say 1000 chars?
sqlite does not care much how you declare your column types and ignores any maximum length specified. The declared type is just a hint; any non-INTEGER PRIMARY KEY column can contain any type.
The size taken up in the database file depends on the values you put in. In the record format, strings are stored as length followed by string data. No empty space is necessarily left there.
I have a large table with say 10 columns. 4 of them remains null most of the times. I have a query that does null value takes any size or no size in bytes. I read few articles some of them are saying :
http://www.sql-server-citation.com/2009/12/common-mistakes-in-sql-server-part-4.html
There is a misconception that if we have the NULL values in a table it doesn't occupy storage space. The fact is, a NULL value occupies space – 2 bytes
SQL: Using NULL values vs. default values
A NULL value in databases is a system value that takes up one byte of storage and indicates that a value is not present as opposed to a space or zero or any other default value.
Can you please guide me regarding the size taken by null value.
If the field is fixed width storing NULL takes the same space as any other value - the width of the field.
If the field is variable width the NULL value takes up no space.
In addition to the space required to store a null value there is also an overhead for having a nullable column. For each row one bit is used per nullable column to mark whether the value for that column is null or not. This is true whether the column is fixed or variable length.
The reason for the discrepancies that you have observed in information from other sources:
The start of the first article is a bit misleading. The article is not talking about the cost of storing a NULL value, but the cost of having the ability to store a NULL (i.e the cost of making a column nullable). It's true that it costs something in storage space to make a column nullable, but once you have done that it takes less space to store a NULL than it takes to store a value (for variable width columns).
The second link seems to be a question about Microsoft Access. I don't know the details of how Access stores NULLs but I wouldn't be surprised if it is different to SQL Server.
The following link claims that if the column is variable length, i.e. varchar then NULL takes 0 bytes (plus 1 byte is used to flag whether value is NULL or not):
How does SQL Server really store NULL-s
The above link, as well as the below link, claim that for fixed length columns, i.e. char(10) or int, a value of NULL occupies the length of the column (plus 1 byte to flag whether it's NULL or not):
Data Type Performance Tuning Tips for Microsoft SQL Server
Examples:
If you set a char(10) to NULL, it occupies 10 bytes (zeroed out)
An int takes 4 bytes (also zeroed out).
A varchar(1 million) set to NULL takes 0 bytes (+ 2 bytes)
Note: on a slight tangent, the storage size of varchar is the length of data entered + 2 bytes.
From this link:
Each row has a null bitmap for columns
that allow nulls. If the row in that
column is null then a bit in the
bitmap is 1 else it's 0.
For variable size datatypes the
acctual size is 0 bytes.
For fixed size datatype the acctual
size is the default datatype size in
bytes set to default value (0 for
numbers, '' for chars).
Storing a NULL value does not take any space.
"The fact is, a NULL value occupies
space – 2 bytes."
This is a misconception -- that's 2 bytes per row, and I'm pretty sure that all rows use those 2 bytes regardless of whether there's any nullable columns.
A NULL value in databases is a system
value that takes up one byte of
storage
This is talking about databases in general, not specifically SQL Server. SQL Server does not use 1 byte to store NULL values.
Even though this questions is specifically tagged as SQL Server 2005, being that it is now 2021, it should be pointed out that it is a "trick question" for any version of SQL Server after 2005.
This is because if either ROW or PAGE compression are used, or if the column is defined as SPARSE, then it will "no space" in the actual row to store a 'NULL value'. These were added in SQL Server 2008.
The implementation notes for ROW COMPRESSION (which is a prerequisite for PAGE COMPRESSION) states:
NULL and 0 values across all data types are optimized and take no bytes1.
While there is still minimal metadata (4 bits per column + (record overhead / columns)) stored per non-sparse column in each physical record2, it's strictly not the value and is required in all cases3.
SPARSE columns with a NULL value take up no space and no relevant per-row metadata (as the number of SPARSE columns increase), albeit with a trade-off for non-NULL values.
As such, it is hard to "count" space without anlyzing the actual DB usage stats. The average bytes per row will vary based on precise column types, table/index rebuild settings, actual data and duplicity, fill capacity, effective page utilization, fragmentation, LOB usage, etc. and is often a more useful metric.
1 SQLite uses a similar approach to have effectively-free NULL values.
2 A brief of the technical layout used in ROW (and thus PAGE) compression can found in "SQL Server 2012 Internals: Special Storage".
Following the 1 or 2 bytes for the number of columns is the CD array, which uses 4 bits [of metadata] for each column in the table to represent information about the length of the column .. 0 (0×0) indicates that the corresponding column is NULL.
3 Fun fact: with ROW compression, bit column values exist entirely in the corresponding 4-bit metadata.
I am using character varying data type in PostgreSQL.
I was not able to find this information in PostgreSQL manual.
What is max limit of characters in character varying data type?
Referring to the documentation, there is no explicit limit given for the varchar(n) type definition. But:
...
In any case, the longest possible
character string that can be stored is
about 1 GB. (The maximum value that
will be allowed for n in the data type
declaration is less than that. It
wouldn't be very useful to change this
because with multibyte character
encodings the number of characters and
bytes can be quite different anyway.
If you desire to store long strings
with no specific upper limit, use text
or character varying without a length
specifier, rather than making up an
arbitrary length limit.)
Also note this:
Tip: There is no performance
difference among these three types,
apart from increased storage space
when using the blank-padded type, and
a few extra CPU cycles to check the
length when storing into a
length-constrained column. While
character(n) has performance
advantages in some other database
systems, there is no such advantage in
PostgreSQL; in fact character(n) is
usually the slowest of the three
because of its additional storage
costs. In most situations text or
character varying should be used
instead.
From documentation:
In any case, the longest possible character string that can be stored is about 1 GB.
character type in postgresql
character varying(n), varchar(n) = variable-length with limit
character(n), char(n) = fixed-length, blank padded
text = variable unlimited length
based on your problem I suggest you to use type text. the type does not require character length.
In addition, PostgreSQL provides the text type, which stores strings of any length. Although the type text is not in the SQL standard, several other SQL database management systems have it as well.
source : https://www.postgresql.org/docs/9.6/static/datatype-character.html
The maximum string size is about 1 GB. Per the postgres docs:
Very long values are also stored in background tables so that they do not interfere with rapid access to shorter column values. In any case, the longest possible character string that can be stored is about 1 GB. (The maximum value that will be allowed for n in the data type declaration is less than that.)
Note that the max n you can specify for varchar is less than the max storage size. While this limit may vary, a quick check reveals that the limit on postgres 11.2 is 10 MB:
psql (11.2)
=> create table varchar_test (name varchar(1073741824));
ERROR: length for type varchar cannot exceed 10485760
Practically speaking, when you do not have a well rationalized length limit, it's suggested that you simply use varchar without specifying one. Per the official docs,
If you desire to store long strings with no specific upper limit, use text or character varying without a length specifier, rather than making up an arbitrary length limit.
I've been trying to estimate the size of an Access table with a certain number of records.
It has 4 Longs (4 bytes each), and a Currency (8 bytes).
In theory: 1 Record = 24 bytes, 500,000 = ~11.5MB
However, the accdb file (even after compacting) increases by almost 30MB (~61 bytes per record). A few extra bytes for padding wouldn't be so bad, but 2.5X seems a bit excessive - even for Microsoft bloat.
What's with the discrepancy? The four longs are compound keys, would that matter?
This is the result of my tests, all conducted with an A2003 MDB, not with A2007 ACCDB:
98,304 IndexTestEmpty.mdb
131,072 IndexTestNoIndexesNoData.mdb
11,223,040 IndexTestNoIndexes.mdb
15,425,536 IndexTestPK.mdb
19,644,416 IndexTestPKIndexes1.mdb
23,838,720 IndexTestPKIndexes2.mdb
24,424,448 IndexTestPKCompound.mdb
28,041,216 IndexTestPKIndexes3.mdb
28,655,616 IndexTestPKCompoundIndexes1.mdb
32,849,920 IndexTestPKCompoundIndexes2.mdb
37,040,128 IndexTestPKCompoundIndexes3.mdb
The names should be pretty self-explanatory, I think. I used an append query with Rnd() to append 524,288 records of fake data, which made the file 11MBs. The indexes I created on the other fields were all non-unique. But if you see the compound 4-column index increased the size from 11MBs (no indexes) to well over 24MBs. A PK on the first column only increased the size only from 11MBs to 15.4MBs (using fake MBs, of course, i.e., like hard drive manufacturers).
Notice how each single-column index added approximately 4MBs to the file size. If you consider that 4 columns with no indexes totalled 11MBs, that seems about right based on my comment above, i.e., that each index should increase the file size by about the amount of data in the field being indexed. I am surprised that the clustered index did this, too -- I thought that the clustered index would use less space, but it doesn't.
For comparison, a non-PK (i.e., non-clustered) unique index on the first column, starting from IndexTestNoIndexes.mdb is exactly the same size as the database with the first column as the PK, so there's no space savings from the clustered index at all. On the off chance that perhaps the ordinal position of the indexed field might make a difference, I also tried a unique index on the second column only, and this came out exactly the same size.
Now, I didn't read your question carefully, and omitted the Currency field, but if I add that to the non-indexed table and the table with the compound index and populate it with random data, I get this:
98,304 IndexTestEmpty.mdb
131,072 IndexTestNoIndexesNoData.mdb
11,223,040 IndexTestNoIndexes.mdb
15,425,536 IndexTestPK.mdb
15,425,536 IndexTestIndexUnique2.mdb
15,425,536 IndexTestIndexUnique1.mdb
15,482,880 IndexTestNoIndexes+Currency.mdb
19,644,416 IndexTestPKIndexes1.mdb
23,838,720 IndexTestPKIndexes2.mdb
24,424,448 IndexTestPKCompound.mdb
28,041,216 IndexTestPKIndexes3.mdb
28,655,616 IndexTestPKCompoundIndexes1.mdb
28,692,480 IndexTestPKCompound+Currency.mdb
32,849,920 IndexTestPKCompoundIndexes2.mdb
37,040,128 IndexTestPKCompoundIndexes3.mdb
The points of comparison are:
11,223,040 IndexTestNoIndexes.mdb
15,482,880 IndexTestNoIndexes+Currency.mdb
24,424,448 IndexTestPKCompound.mdb
28,692,480 IndexTestPKCompound+Currency.mdb
So, the currency field added another 4.5MBs, and its index added another 4MBs. And if I add non-unique indexes to the 2nd, 3rd and 4th long fields, the database 41,336,832, and increase in size of just under 12MBs (or ~4MBs per additional index).
So, this basically replicates your results, no? And I ended up with the same file sizes, roughly speaking.
The answer to your question is INDEXES, though there is obviously more overhead in the A2007 ACCDB format, since I saw an increase in size of only 20MBs, not 30MBs.
One thing I did notice was that I could implement an index that would make the file larger, then delete the index and compact, and it would return to exactly the same file size as it had before, so you should be able to take a single copy of your database and experiment with what removing the indexes does to your file size.