I want to know the size of my Table (postgresql). I make this query:
select pg_size_pretty(pg_table_size('mytable'));
Result: 8192 bytes
Then, I add 4 rows and the result is the same (8192 bytes).
What am I doing wrong? What am I missing?
Thanks a lot...
Postgres puts records in fixed-size pages, which are 8kB each by default. Storage is allocated one page at a time. Once you add enough rows to reach your table's fillfactor, it will add a second block, and the size will jump to 16384 bytes.
Related
We have table "table1" containing 100000 rows, which includes blob data. The total table size is 3.5 GB.
When tried to extract all records into CSV/.txt file by using copy or \o commands the generated output file is 100 GB in size.
ex:
select pg_size_pretty(pg_total_relation_size('table1));
3.5 GB
Please let us know how this actually generated this much huge size and how will get know the actual table size?
I am exploring storage mechanism of postgres. I know that postgres is using page like structure(each of size 8K) to store rows. One page can contain more than one row. I also know that TOASTing is done by postgres, when the row can not be contained in given page.
But I am not certain about following scerio :-
There's only 1K space left in current page, and the size of newly created row exceeds one 1K. In that case, what will happen ? Will new page be allocated for that row and old page will have unused space ? OR the old page's remaining space will be occupied, when another row with size less than or equal to 1K is created ?
I am referring TOAST. Following para is bit unclear :-
When a row that is to be stored is "too wide" (the threshold for that is 2KB by default), the TOAST mechanism first attempts to compress any wide field values. If that isn't enough to get the row under 2KB, it breaks up the wide field values into chunks that get stored in the associated TOAST table. Each original field value is replaced by a small pointer that shows where to find this "out of line" data in the TOAST table. TOAST will attempt to squeeze the user-table row down to 2KB in this way, but as long as it can get below 8KB, that's good enough and the row can be stored successfully.
Why it's talking about two sizes 8K and 2K ? Why postgres checks for threshold 2K ?
Thanks in advance.
First, I should clarify that “enough room in the table page” has nothing to do with the question if an attribute is TOASTed or not.
The paragraph you quote describes how TOAST tries to reduce the size of a table row that exceeds 2KB by first compressing the values and then storing them “out of line” in a TOAST table.
The idea is to reduce the size such that a row does not use up more than a quarter of the space in a table block. But if that fails, and the row ends up bigger than 2KB after TOASTing, that is no problem either, as long if the resulting row fits into one 8KB block.
A table row is always stored in a single table block. If there is not enough space left in any existing block, a new table block is allocated and the existing blocks are left with some empty space. This empty space can still be used for other, smaller new rows.
The limits of 8KB for a table block and 2KB for the threshold for TOASTing are somewhat arbitrary and based on experience. You can change them if you are ready to recompile PostgreSQL (from PostgreSQL v11 on, you can specify the block size when you create the database cluster with initdb), but I have not heard any reports that this is a good idea.
In PostgreSQL, how can I tell whether a text column is stored inline or stored in a "background table"?
Documentation for text column types says that
Very long values are also stored in background tables so that they do not interfere with rapid access to shorter column values.
Is there a fixed length at which a value is determined to be "very long"? If not, are there other ways of telling how my columns are laid out on disk? I have a table with several columns that are text (or varchar(n)) and want to understand how they are stored under the hood. Is there more documentation on these "background tables" somewhere?
Any varlena data type (all types with variable length or types longer than 4 bytes (32 bits) or 8 bytes (64 bits)) can be TOASTed - TOAST is a process that tries to reduce long rows (records) to 8KB page size.
Row size is checked before physically storing to the relation. When the size exceeds 2KB, most larger fields are selected, compressed, sliced to 2KB chunks and moved to a secondary table file with the suffix _toast. A pointer to the toast file replaces the data in the main storage. This process is repeated while the row is bigger than 2KB.
Follow the links provided by a_horse_with_no_name and IMSoP for more detailed documentation.
If your table is called t1, then enter \d+ t1 at your psql prompt, it will show a column storage mode.
Apparently PostgreSQL stores a couple of values in the header of each database row.
If I don't use NULL values in that table - is the null bitmap still there?
Does defining the columns with NOT NULL make any difference?
It's actually more complex than that.
The null bitmap needs one bit per column in the row, rounded up to full bytes. It is only there if the actual row includes at least one NULL value and is fully allocated in that case. NOT NULL constraints do not directly affect that. (Of course, if all fields of your table are NOT NULL, there can never be a null bitmap.)
The "heap tuple header" (per row) is 23 bytes long. Actual data starts at a multiple of MAXALIGN (Maximum data alignment) after that, which is typically 8 bytes on 64-bit OS (4 bytes on 32-bit OS). Run the following command from your PostgreSQL binary dir as root to get a definitive answer:
./pg_controldata /path/to/my/dbcluster
On a typical Debian-based installation of Postgres 12 that would be:
sudo /usr/lib/postgresql/12/bin/pg_controldata /var/lib/postgresql/12/main
Either way, there is one free byte between the header and the aligned start of the data, which the null bitmap can utilize. As long as your table has 8 columns or less, NULL storage is effectively absolutely free (as far as disk space is concerned).
After that, another MAXALIGN (typically 8 bytes) is allocated for the null bitmap to cover another (typically) 64 fields. Etc.
This is valid for at least versions 8.4 - 12 and most likely won't change.
The null bitmap is only present if the HEAP_HASNULL bit is set in t_infomask. If it is present it begins just after the fixed header and occupies enough bytes to have one bit per data column (that is, t_natts bits altogether). In this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When the bitmap is not present, all columns are assumed not-null.
http://www.postgresql.org/docs/9.0/static/storage-page-layout.html#HEAPTUPLEHEADERDATA-TABLE
so for every 8 columns you use one byte of extra storage. Then for every about million rows that would take up one megabyte of storage. Does not really seem that important. I would define the tables how they needed to be defined and not worry about null headers.
Is it possible to predict the amount of disk space/memory that will be used by a basic index in PostgreSQL 9.0?
E.g. If I have a default B-tree index on an integer column in a table of 1 million rows, how much space would be taken up by the index? Is the entire index held in memory at all times?
Not really a definitive answer, but I looked at a table in a 9.0 test system I have with a couple of int indexes on a table of 280k rows. The indexs all report a size of 6232kb. So roughly 22 bytes per row.
There is no way to say that. It depends on the type of operations you will make, as PostgreSQL stores many different versions of the same row, including row versions stored in index files.
Just make the table you are interested in and check it.