Indexing Engine: Index Write Overhead
To convey the cost of maintaining existing and proposed indexes, we define the Index Write Overhead metric as an estimate of the I/O impact of maintaining that index. This can guide you to consolidate indexes and drop little-used indexes to have more I/O headroom. This estimate will be available for both existing indexes, as well as new suggested indexes.
To understand this metric, it's important to understand the basics of how indexes are maintained in Postgres. When data is written to a table, indexes referencing that data may also need to be updated. Generally, indexes must be updated for inserts and updates, and when the table is VACUUMed (deletes are "free", but do incur a cost during VACUUM).
The size of an index entry for a particular indexed value will depend on the index type and the columns being indexed (plus any columns added with INCLUDE).
When considering index write overhead in aggregate, index storage parameters
(in Postgres 13+), and bottom-up index deletion (in Postgres 14+) can also
affect the frequency of index writes. Similarly, partial indexes will only
need to be updated for changes to rows matching their index predicate. Finally,
indexes do not need to be touched for HOT updates.
To give a sense of the impact of writes, pganalyze models a simplified version of the above mechanism. The metric is defined as "bytes written to the index per byte written to the table": for every byte written to the table, this many bytes must be written to the index.
The actual calculation is straightforward, though it involves some Postgres internals. The basic formula for overhead is:
write overhead = index entry size / row size * partial index selectivity
The index entry size is an 8-byte header plus the average width (based on column stats if available; otherwise using a generic estimate based on data type) of all indexed columns.
The row size is a 23-byte header, plus a 4-byte item pointer, plus the average width (again, based on column stats if available) of all the columns in the table.
The partial index selectivity is 1 for normal indexes (without a
clause), and generically estimated at 10% for partial indexes.
Let's walk through a simple example. Let's say you have a schema like
CREATE TABLE users ( id bigserial primary key, email text, password_hash bytea, organization_id bigint, team_id bigint ); CREATE INDEX ON users (organization_id);
To estimate the write overhead for the organization_id index, first we would look at the table stats. Say the column stats show the following average column widths:
id = 8 email = 20 password_hash = 60 organization_id = 8 team_id = 8
With this, we can calculate the index entry size as
8 bytes (header) + 8 bytes (organization_id) + 8 bytes (team_id) = 24 bytes
Similarly, the row size is
23 bytes (header) + 4 bytes (item pointer) + 8 + 20 + 60 + 8 + 8 (all columns) = 131 bytes
Since this is not a partial index, the selectivity is 1.
The total index write overhead for this index is
24 / 131 * 1 ≈ 0.183
For every byte written to the table, approximately 0.183 bytes will be written to update this index.
Unfortunately, there's no one right value for write overhead. Whether the number from the example above is "good" or "bad" depends on several factors:
- how much I/O headroom does the database have?
- how frequently is this table written to? (excluding HOT updates which do not need to touch indexes)
- how important is the latency for queries relying on this index (and how slow would they be without it)?
In general, it's useful to think of write overhead as a form of backpressure against the temptation to just index everything. It's a way to convey that indexing has a cost beyond just the disk space used, and to approximate that cost as you compare different indexes.
Though Index Write Overhead is a useful metric, it's important to understand it's based on heuristics and simplifications. Here are the issues to be aware of:
- currently only supported for b-tree indexes
- uses a coarse heuristic for expression indexes (always assumes 8 bytes for expression result width)
- INCLUDEd columns are not accounted for
- partial index selectivity is always estimated at a generic 10%
- NULL index entries are not sized correctly
- memory alignment issues are ignored
- compression of values is ignored
- non-leaf pages in b-tree indexes are ignored (this is a reasonable approximation for even moderately large indexes)
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