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Data Alignment in PostgreSQL

Kuntal Ghosh12/6/2018
Monitoring & tuning

When data are naturally aligned, CPU can perform read and write to memory efficiently. Hence, each data type in PostgreSQL has a specific alignment requirement. When multiple attributes are stored consecutively in a tuple, padding is inserted before an attribute so that it begins from the required aligned boundary. A better understanding of these alignment requirements may help minimizing the amount of padding required while storing a tuple on disk, thus saving disk space.

 

Data types in Postgres are divided into following categories:

  1. 1. Pass-by-value, fixed length: Data types that are passed by values to Postgres internal routines and have fixed lengths fall into this category.. The length can be 1, 2,  or 4 (or 8 on 64-bit systems) bytes.
  2. 2. Pass-by-reference, fixed length: For these data types, an address reference from the in-memory heap page is sent to internal Postgres routines. They also have fixed lengths.
  3. 3. Pass-by_reference, variable length: For variable length data types, Postgres prepends a varlena header before the actual data. It stores some information about how the data is actually stored on-disk (uncompressed, compressed or TOASTed) and the actual length of the data. For TOASTed attributes, the actual data is stored in a separate relation. In these cases, the varlena headers follow some information about the actual location of the data in their corresponding TOAST relation.

 Typically, on-disk size of a varlena header is 1-byte. But, if the data cannot be toasted and size of the uncompressed data crosses 126 bytes, it uses a 4-bytes header. For example,

CREATE TABLE t1 (
 , a varchar
);
insert into t1 values(repeat('a',126));
insert into t1 values(repeat('a',127));
select pg_column_size(a) from t1;
pg_column_size
---------------------
          127 
          131 

 Besides, attributes having 4-bytes varlena header need to be aligned to a 4-bytes aligned memory location. It may waste upto 3-bytes of additional padding space. So, some careful length restrictions on such columns may save space.

        4. Pass-by_reference, variable length(cstring, unknown): Finally, there are two data types, viz. ctring and unknown, which are string literals. They can be stored from any 1-byte aligned boundary. Also, they do not require any varlena header.

You can check the alignment requirements of each type using the following query,

select typname,typbyval,typlen,typalign from pg_type;

Where typname is name of the data type; typbyval is true if the data type is accessed as passed-by-value, else false; typlen is the actual length of the data type, however for data-types of variable length it has a value < 0 (-2 for cstring and unknown, -1 otherwise); typalign is the required alignment for the data type.

After checking the alignment requirement of each data type, you can reduce the padding space by positioning them in their alignment-favourable way. For example, following table schema wastes a lot of disk space for padding:

CREATE TABLE t1 (
 , a char
 , b int2    -- 1 byte of padding after a
 , c char
 , d int4    -- 3 bytes of padding after c
 , e char
 , f int8    -- 7 bytes of padding after e
 );

If you reorder the columns as double aligned column first, int aligned, short aligned and char aligned column in last, you can save (1+3+7)=11 bytes of space per tuple. 

CREATE TABLE t1 (
 , f int8
 , d int4
 , b int2
 , a char
 , c char
 , e char
 );

Before the tuple attributes, a fixed size 23-byte tuple header followed by an optional null-bitmap and an optional object ID are stored. The attributes always starts from a MAXALIGN-ed boundary - typically 8 bytes on a 64-bit OS (or 4 bytes on a 32-bit OS). So, the effective size of a minimal tuple header is 24-bytes (23-bytes header + 1-byte padding). When the null-bitmap is present, it occupies enough bytes to have one bit per data column. 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. For example[3],

SELECT pg_column_size(row(NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL));
pg_column_size
----------------
            24

This includes the 23-bytes fixed-sized tuple header and a null-bitmap consisting of 8-bits. Now, if we increase the number of columns to nine, it includes 23-bytes fixed-sized tuple header, a null-bitmap consisting of 16-bits and 7 bytes of padding.

Saving a few bytes per tuple in a relation consisting of millions of rows may result in saving some significant storage space. Besides, if we can fit more tuple in a data page, performance can be improved due to lesser I/O activity.

References:

[1] https://www.postgresql.org/docs/current/static/catalog-pg-type.html

[2] https://www.postgresql.org/docs/current/static/storage-page-layout.html

[3] https://www.enterprisedb.com/blog/column-storage-intervals