143 lines
6.5 KiB
Markdown
143 lines
6.5 KiB
Markdown
# Ordering Table Columns in PostgreSQL
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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 following column. Let's consider an 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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The first column is a 4-byte integer. The next is text of variable length. The
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`text` data type requires 1-word alignment, and on 64-bit platform, 1 word is 8
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bytes. To meet the alignment requirements, four zeros are to be added right
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after the first column, so `id` occupies 4 bytes, then 4 bytes of alignment
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padding, and only next `name` is being stored. Therefore, in this case, 8 bytes
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will be spent for storing a 4-byte integer.
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The space between rows is also subject to alignment padding. The `user_id`
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column takes only 4 bytes, and on 64-bit platform, 4 zeroes will be added for
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alignment padding, to allow storing the next row beginning with the "clear" word.
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As a result, the actual size of each column would be (ommiting variable length
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data and 24-byte tuple header): 8 bytes, variable, 8 bytes. This means that
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each row will require at least 16 bytes for the two 4-byte integers. If a table
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has a few rows this is not an issue. However, once you start storing millions of
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rows you can save space by using a different order. For the above example, the
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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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or
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- `name` (text, variable)
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- `id` (integer, 4 bytes)
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- `user_id` (integer, 4 bytes)
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In these examples, the `id` and `user_id` columns are packed together, which
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means we only need 8 bytes to store _both_ of them. This in turn means each row
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will require 8 bytes less 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 (`text`, `varchar`, arrays,
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`json`, `jsonb`, and so on) at the end.
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## Type Sizes
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While the [PostgreSQL documentation](https://www.postgresql.org/docs/current/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 | Alignment needed |
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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 and tuple header, we need at
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least 8 * 6 = 48 bytes per 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 and 24-byte
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tuple header. 8 bytes being saved may not sound like much, but for tables as
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large as the `events` table it does begin to matter. For example, when storing
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80 000 000 rows this translates to a space saving of at least 610 MB, all by
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just changing the order of a few columns.
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