127 lines
5.7 KiB
Markdown
127 lines
5.7 KiB
Markdown
# Ordering Table Columns
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Similar to C structures the space of a table is influenced by the order of
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columns. This is because the size of columns is aligned depending on the type of
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the column. Take the following column order for example:
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* id (integer, 4 bytes)
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* name (text, variable)
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* user_id (integer, 4 bytes)
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Integers are aligned to the word size. This means that on a 64 bit platform the
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actual size of each column would be: 8 bytes, variable, 8 bytes. This means that
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each row will require at least 16 bytes for the two integers, and a variable
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amount for the text field. If a table has a few rows this is not an issue, but
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once you start storing millions of rows you can save space by using a different
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order. For the above example a more ideal column order would be the following:
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* id (integer, 4 bytes)
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* user_id (integer, 4 bytes)
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* name (text, variable)
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In this setup the `id` and `user_id` columns can be packed together, which means
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we only need 8 bytes to store _both_ of them. This in turn each row will require
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8 bytes less of space.
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For GitLab we require that columns of new tables are ordered based to use the
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least amount of space. An easy way of doing this is to order them based on the
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type size in descending order with variable sizes (string and text columns for
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example) at the end.
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## Type Sizes
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While the PostgreSQL docuemntation
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(https://www.postgresql.org/docs/current/static/datatype.html) contains plenty
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of information we will list the sizes of common types here so it's easier to
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look them up. Here "word" refers to the word size, which is 4 bytes for a 32
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bits platform and 8 bytes for a 64 bits platform.
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| Type | Size | Aligned To |
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|:-----------------|:-------------------------------------|:-----------|
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| smallint | 2 bytes | 1 word |
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| integer | 4 bytes | 1 word |
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| bigint | 8 bytes | 8 bytes |
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| real | 4 bytes | 1 word |
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| double precision | 8 bytes | 8 bytes |
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| boolean | 1 byte | not needed |
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| text / string | variable, 1 byte plus the data | 1 word |
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| bytea | variable, 1 or 4 bytes plus the data | 1 word |
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| timestamp | 8 bytes | 8 bytes |
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| timestamptz | 8 bytes | 8 bytes |
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| date | 4 bytes | 1 word |
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A "variable" size means the actual size depends on the value being stored. If
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PostgreSQL determines this can be embedded directly into a row it may do so, but
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for very large values it will store the data externally and store a pointer (of
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1 word in size) in the column. Because of this variable sized columns should
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always be at the end of a table.
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## Real Example
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Let's use the "events" table as an example, which currently has the following
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layout:
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| Column | Type | Size |
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|:------------|:----------------------------|:---------|
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| id | integer | 4 bytes |
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| target_type | character varying | variable |
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| target_id | integer | 4 bytes |
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| title | character varying | variable |
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| data | text | variable |
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| project_id | integer | 4 bytes |
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| created_at | timestamp without time zone | 8 bytes |
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| updated_at | timestamp without time zone | 8 bytes |
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| action | integer | 4 bytes |
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| author_id | integer | 4 bytes |
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After adding padding to align the columns this would translate to columns being
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divided into fixed size chunks as follows:
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| Chunk Size | Columns |
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|:-----------|:------------------|
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| 8 bytes | id |
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| variable | target_type |
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| 8 bytes | target_id |
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| variable | title |
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| variable | data |
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| 8 bytes | project_id |
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| 8 bytes | created_at |
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| 8 bytes | updated_at |
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| 8 bytes | action, author_id |
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This means that excluding the variable sized data we need at least 48 bytes per
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row.
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We can optimise this by using the following column order instead:
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| Column | Type | Size |
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|:------------|:----------------------------|:---------|
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| created_at | timestamp without time zone | 8 bytes |
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| updated_at | timestamp without time zone | 8 bytes |
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| id | integer | 4 bytes |
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| target_id | integer | 4 bytes |
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| project_id | integer | 4 bytes |
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| action | integer | 4 bytes |
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| author_id | integer | 4 bytes |
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| target_type | character varying | variable |
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| title | character varying | variable |
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| data | text | variable |
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This would produce the following chunks:
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| Chunk Size | Columns |
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|:-----------|:-------------------|
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| 8 bytes | created_at |
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| 8 bytes | updated_at |
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| 8 bytes | id, target_id |
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| 8 bytes | project_id, action |
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| 8 bytes | author_id |
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| variable | target_type |
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| variable | title |
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| variable | data |
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Here we only need 40 bytes per row excluding the variable sized data. 8 bytes
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being saved may not sound like much, but for tables as large as the "events"
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table it does begin to matter. For example, when storing 80 000 000 rows this
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translates to a space saving of at least 610 MB: all by just changing the order
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of a few columns.
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