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34 lines
1.1 KiB
Markdown
34 lines
1.1 KiB
Markdown
# Include Columns In A Covering Index
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A _covering index_ is a special type of B-Tree index that, in addition to
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indexing on a certain field, also _includes_ one or more columns as extra data
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in the leaves of the tree. When created correctly, this can speed up the
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queries it targets by achieving an _index-only scan_.
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Let's say we have a frequently run query on a large `events` table that looks
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like this:
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```sql
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select user_id, identifier, type
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from events
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where user_id = $1;
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```
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Here is what it looks like to create an index for this query with the `include`
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keyword:
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```sql
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create index user_id_on_events_idx
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on (user_id)
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include (identifier, type);
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```
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An index on its own can already cause a significant speed up to the queries it
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targets, but may still need to retrieve some `select` attributes from the
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table. For hot-path queries with a set of specific columns always included in
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the select, there can be significant additional speed ups by having the index
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_cover_ those columns.
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For more details, check out [A Close Look At The Index Include
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Clause](https://use-the-index-luke.com/blog/2019-04/include-columns-in-btree-indexes).
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