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diff --git a/doc/development/database/ordering_table_columns.md b/doc/development/database/ordering_table_columns.md new file mode 100644 index 00000000000..7cd3d4fb208 --- /dev/null +++ b/doc/development/database/ordering_table_columns.md @@ -0,0 +1,152 @@ +--- +stage: Data Stores +group: Database +info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments +--- + +# Ordering Table Columns in PostgreSQL + +For GitLab we require that columns of new tables are ordered to use the +least amount of space. An easy way of doing this is to order them based on the +type size in descending order with variable sizes (`text`, `varchar`, arrays, +`json`, `jsonb`, and so on) at the end. + +Similar to C structures the space of a table is influenced by the order of +columns. This is because the size of columns is aligned depending on the type of +the following column. Let's consider an example: + +- `id` (integer, 4 bytes) +- `name` (text, variable) +- `user_id` (integer, 4 bytes) + +The first column is a 4-byte integer. The next is text of variable length. The +`text` data type requires 1-word alignment, and on 64-bit platform, 1 word is 8 +bytes. To meet the alignment requirements, four zeros are to be added right +after the first column, so `id` occupies 4 bytes, then 4 bytes of alignment +padding, and only next `name` is being stored. Therefore, in this case, 8 bytes +are spent for storing a 4-byte integer. + +The space between rows is also subject to alignment padding. The `user_id` +column takes only 4 bytes, and on 64-bit platform, 4 zeroes are added for +alignment padding, to allow storing the next row beginning with the "clear" word. + +As a result, the actual size of each column would be (omitting variable length +data and 24-byte tuple header): 8 bytes, variable, 8 bytes. This means that +each row requires at least 16 bytes for the two 4-byte integers. If a table +has a few rows this is not an issue. However, once you start storing millions of +rows you can save space by using a different order. For the above example, the +ideal column order would be the following: + +- `id` (integer, 4 bytes) +- `user_id` (integer, 4 bytes) +- `name` (text, variable) + +or + +- `name` (text, variable) +- `id` (integer, 4 bytes) +- `user_id` (integer, 4 bytes) + +In these examples, the `id` and `user_id` columns are packed together, which +means we only need 8 bytes to store _both_ of them. This in turn means each row +requires 8 bytes less space. + +Since Ruby on Rails 5.1, the default data type for IDs is `bigint`, which uses 8 bytes. +We are using `integer` in the examples to showcase a more realistic reordering scenario. + +## Type Sizes + +While the [PostgreSQL documentation](https://www.postgresql.org/docs/current/datatype.html) contains plenty +of information we list the sizes of common types here so it's easier to +look them up. Here "word" refers to the word size, which is 4 bytes for a 32 +bits platform and 8 bytes for a 64 bits platform. + +| Type | Size | Alignment needed | +|:-----------------|:-------------------------------------|:-----------| +| `smallint` | 2 bytes | 1 word | +| `integer` | 4 bytes | 1 word | +| `bigint` | 8 bytes | 8 bytes | +| `real` | 4 bytes | 1 word | +| `double precision` | 8 bytes | 8 bytes | +| `boolean` | 1 byte | not needed | +| `text` / `string` | variable, 1 byte plus the data | 1 word | +| `bytea` | variable, 1 or 4 bytes plus the data | 1 word | +| `timestamp` | 8 bytes | 8 bytes | +| `timestamptz` | 8 bytes | 8 bytes | +| `date` | 4 bytes | 1 word | + +A "variable" size means the actual size depends on the value being stored. If +PostgreSQL determines this can be embedded directly into a row it may do so, but +for very large values it stores the data externally and store a pointer (of +1 word in size) in the column. Because of this variable sized columns should +always be at the end of a table. + +## Real Example + +Let's use the `events` table as an example, which currently has the following +layout: + +| Column | Type | Size | +|:--------------|:----------------------------|:---------| +| `id` | integer | 4 bytes | +| `target_type` | character varying | variable | +| `target_id` | integer | 4 bytes | +| `title` | character varying | variable | +| `data` | text | variable | +| `project_id` | integer | 4 bytes | +| `created_at` | timestamp without time zone | 8 bytes | +| `updated_at` | timestamp without time zone | 8 bytes | +| `action` | integer | 4 bytes | +| `author_id` | integer | 4 bytes | + +After adding padding to align the columns this would translate to columns being +divided into fixed size chunks as follows: + +| Chunk Size | Columns | +|:-----------|:----------------------| +| 8 bytes | `id` | +| variable | `target_type` | +| 8 bytes | `target_id` | +| variable | `title` | +| variable | `data` | +| 8 bytes | `project_id` | +| 8 bytes | `created_at` | +| 8 bytes | `updated_at` | +| 8 bytes | `action`, `author_id` | + +This means that excluding the variable sized data and tuple header, we need at +least 8 * 6 = 48 bytes per row. + +We can optimise this by using the following column order instead: + +| Column | Type | Size | +|:--------------|:----------------------------|:---------| +| `created_at` | timestamp without time zone | 8 bytes | +| `updated_at` | timestamp without time zone | 8 bytes | +| `id` | integer | 4 bytes | +| `target_id` | integer | 4 bytes | +| `project_id` | integer | 4 bytes | +| `action` | integer | 4 bytes | +| `author_id` | integer | 4 bytes | +| `target_type` | character varying | variable | +| `title` | character varying | variable | +| `data` | text | variable | + +This would produce the following chunks: + +| Chunk Size | Columns | +|:-----------|:-----------------------| +| 8 bytes | `created_at` | +| 8 bytes | `updated_at` | +| 8 bytes | `id`, `target_id` | +| 8 bytes | `project_id`, `action` | +| 8 bytes | `author_id` | +| variable | `target_type` | +| variable | `title` | +| variable | `data` | + +Here we only need 40 bytes per row excluding the variable sized data and 24-byte +tuple header. 8 bytes being saved may not sound like much, but for tables as +large as the `events` table it does begin to matter. For example, when storing +80 000 000 rows this translates to a space saving of at least 610 MB, all by +just changing the order of a few columns. |