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diff --git a/doc/development/ordering_table_columns.md b/doc/development/ordering_table_columns.md new file mode 100644 index 00000000000..249e70c7b0e --- /dev/null +++ b/doc/development/ordering_table_columns.md @@ -0,0 +1,127 @@ +# Ordering Table Columns + +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 column. Take the following column order for example: + +* id (integer, 4 bytes) +* name (text, variable) +* user_id (integer, 4 bytes) + +Integers are aligned to the word size. This means that on a 64 bit platform the +actual size of each column would be: 8 bytes, variable, 8 bytes. This means that +each row will require at least 16 bytes for the two integers, and a variable +amount for the text field. If a table has a few rows this is not an issue, but +once you start storing millions of rows you can save space by using a different +order. For the above example a more ideal column order would be the following: + +* id (integer, 4 bytes) +* user_id (integer, 4 bytes) +* name (text, variable) + +In this setup the `id` and `user_id` columns can be packed together, which means +we only need 8 bytes to store _both_ of them. This in turn each row will require +8 bytes less of space. + +For GitLab we require that columns of new tables are ordered based 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 (string and text columns for +example) at the end. + +## Type Sizes + +While the PostgreSQL docuemntation +(https://www.postgresql.org/docs/current/static/datatype.html) contains plenty +of information we will 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 | Aligned To | +|:-----------------|:-------------------------------------|:-----------| +| 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 will store 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 we need at least 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. 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. |