let's say I have a column on my table defined the following:
"MyColumn" smallint NULL
Storing a value like 0, 1 or something else should need 2 bytes (1). But how much space is needed if I set "MyColumn" to NULL? Will it need 0 bytes?
Are there some additional needed bytes for administration purpose or such things for every column/row?
(1) http://www.postgresql.org/docs/9.0/interactive/datatype-numeric.html
Laramie is right about the bitmap and links to the right place in the manual. Yet, this is almost, but not quite correct:
So for any given row with one or more nulls, the size added to it
would be that of the bitmap(N bits for an N-column table, rounded up).
One has to factor in data alignment. The HeapTupleHeader (per row) is 23 bytes long, actual column data always starts at a multiple of MAXALIGN (typically 8 bytes). That leaves one byte of padding that can be utilized by the null bitmap. In effect NULL storage is absolutely free for tables up to 8 columns.
After that, another MAXALIGN (typically 8) bytes are allocated for the next MAXALIGN * 8(typically 64) columns. Etc. Always for the total number of user columns (all or nothing). But only if there is at least one actual NULL value in the row.
I ran extensive tests to verify all of that. More details:
Does not using NULL in PostgreSQL still use a NULL bitmap in the header?
Null columns are not stored. The row has a bitmap at the start and one bit per column that indicates which ones are null or non-null. The bitmap could be omitted if all columns are non-null in a row. So for any given row with one or more nulls, the size added to it would be that of the bitmap(N bits for an N-column table, rounded up).
More in depth discussion from the docs here
It should need 1 byte (0x00) however it's the structure of the table that makes up most of the space, adding this one value might change something (Like adding a row) which needs more space than the sum of the data in it.
Edit: Laramie seems to know more about null than me :)
Related
Should I define a column type from actual length to nth power of 2?
The first case, I have a table column store no more than 7 charactors,
will I use NVARCHAR(8)? since there maybe implicit convert inside Sql
server, allocate 8 space and truncate automatic(heard some where).
If not, NCHAR(7)/NCHAR(8), which should be(assume the fixed length is 7)
Any performance differ on about this 2 cases?
You should use the actual length of the string. Now, if you know that the value will always be exactly 7 characters, then use CHAR(7) rather than VARCHAR(7).
The reason you see powers-of-2 is for columns that have an indeterminate length -- a name or description that may not be fixed. In most databases, you need to put in some maximum length for the varchar(). For historical reasons, powers-of-2 get used for such things, because of the binary nature of the underlying CPUs.
Although I almost always use powers-of-2 in these situations, I can think of no real performance differences. There is one. . . in some databases the actual length of a varchar(255) is stored using 1 byte whereas a varchar(256) uses 2 bytes. That is a pretty minor difference -- even when multiplied over millions of rows.
Assuming I have a table in PostgreSQL, how can I find the exact byte size used by the system in order to save a specific row of my table?
For example, assume I have a table with a VARCHAR(1000000) field and some rows contain really big strings for this field while others really small. How can I check the byte size of a row in this case? (including the byte size even in the case TOAST is being used).
Use pg_column_size and octet_length.
See:
How can pg_column_size be smaller than octet_length?
How can I find out how big a large TEXT field is in Postgres?
How can pg_column_size be smaller than octet_length?
I would like to know how do i convert number of row into size like in MB or KB?
Is there a way to do that or formula?
The reason I doing this is because I would like to know given this set of data but not all in tablespace, how much size is used by this set of data.
Thanks,
keith
If you want an estimate, you could multiple the row count with the information from user_table.avg_row_len for that table.
If you want the real size of the table on disk, this is available user_segments.bytes. Note that the smallest unit Oracle will use is a block. So even for an empty table, you will see a value that is bigger tzen zero in that column. That is actual size of the space reserved in the tablespace for that table.
So SQL server places a limit of 900 bytes on an index. I have columns that are NVARCHAR(1000) that I need to search on. Full text search of these columns is not required because search will always occur on the complete value or a prefix of the complete value. I will never need to search for terms that lie in the middle/end of the value.
The rows of the tables in question will never be updated predicated on this index, and the actual values that exceed 450 chars are outliers that will never be searched for.
Given the above, is there any reason not to ignore the warning:
The total size of an index or primary key cannot exceed 900 bytes
?
We shouldn't ignore the warning as any subsequent INSERT or UPDATE statement that specifies data values that generates a key value longer than 900 bytes will fail. Following link might help:
http://decipherinfosys.wordpress.com/2007/11/06/the-900-byte-index-limitation-in-sql-server/
i have created a table and 1 of the columns in it is float (named ratio),
also have 2x int columns of type INT (KILL and DEATH), ratio column updated automatically by trigger (each time KILL or DEATH is updated the ratio is updated by the trigger),
the size of the float column (ratio) is too big, i mean its length is too long, how i can make define the size of a float column by default?
thanks in advance,
The ratio between two columns is something you ought to be calculating on the fly, as you query the tables. Using a trigger is overkill, IMHO. You could use a view or a calculated column if you are concerned about repeating your logic (such as avoiding division by zero) over and over.
Formatting of a column (how many decimal places) is an application or report issue, not so much a database issue. If at some point you decide that you actually want more precision in one of several displays, you'll have to make database changes rather than just app changes. You also might have a problem if you ever had a ratio of 1 in 1000 or smaller: if you limit yourself to 3 decimal places, your ratio will be calculated as 0, which might cause problems in your logic.