Cost-Based Optimizer

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The cost-based optimizer seeks the lowest cost for a query, usually related to time.

How is cost calculated?

A given SQL query can have thousands of equivalent query plans with vastly different execution times. The cost-based optimizer enumerates these plans and chooses the lowest cost plan.

Cost is roughly calculated by:

Estimating how much time each node in the query plan will use to process all results
Modeling how data flows through the query plan

The most important factor in determining the quality of a plan is cardinality (i.e., the number of rows); the fewer rows each SQL operator needs to process, the faster the query will run.

Table statistics

The cost-based optimizer can often find more performant query plans if it has access to statistical data on the contents of your tables. This data needs to be generated from scratch for new tables, and regenerated periodically for existing tables.

By default, CockroachDB automatically generates table statistics when tables are created, and as they are updated. It does this using a background job that automatically determines which columns to get statistics on — specifically, it chooses:

Columns that are part of the primary key or an index (in other words, all indexed columns).
Up to 100 non-indexed columns.

By default, CockroachDB also automatically collects multi-column statistics on columns that prefix an index.

Note:

Schema changes trigger automatic statistics collection for the affected table(s).

Control automatic statistics

For best query performance, most users should leave automatic statistics enabled with the default settings. The information provided in this section is useful for troubleshooting or performance tuning by advanced users.

Control statistics refresh rate

Statistics are refreshed in the following cases:

When there are no statistics.
When it's been a long time since the last refresh, where "long time" is defined according to a moving average of the time across the last several refreshes.
After a successful IMPORT or RESTORE into the table.
After any schema change affecting the table.

After each mutation operation (INSERT, UPDATE, or DELETE), the probability of a refresh is calculated using a formula that takes the cluster settings shown in the following table as inputs. These settings define the target number of rows in a table that should be stale before statistics on that table are refreshed. Increasing either setting will reduce the frequency of refreshes. In particular, min_stale_rows impacts the frequency of refreshes for small tables, while fraction_stale_rows has more of an impact on larger tables.

Setting	Default Value	Details
`sql.stats.automatic_collection.fraction_stale_rows`	0.2	Target fraction of stale rows per table that will trigger a statistics refresh
`sql.stats.automatic_collection.min_stale_rows`	500	Target minimum number of stale rows per table that will trigger a statistics refresh

Note:

Because the formula for statistics refreshes is probabilistic, you will not see statistics update immediately after changing these settings, or immediately after exactly 500 rows have been updated.

Turn off statistics

To turn off automatic statistics collection, follow these steps:

Run the following statement to disable the automatic statistics cluster setting:
```
> SET CLUSTER SETTING sql.stats.automatic_collection.enabled = false;
```
Use the SHOW STATISTICS statement to view automatically generated statistics.
Delete the automatically generated statistics using the following statement:
```
> DELETE FROM system.table_statistics WHERE true;
```
Restart the nodes in your cluster to clear the statistics caches.

To see how to manually generate statistics, see the CREATE STATISTICS examples.

Control histogram collection

By default, the optimizer collects histograms for all index columns (specifically the first column in each index) during automatic statistics collection. If a single column statistic is explicitly requested using manual invocation of CREATE STATISTICS, a histogram will be collected, regardless of whether or not the column is part of an index.

Note:

CockroachDB does not support histograms on ARRAY-typed columns. As a result, statistics created on ARRAY-typed columns do not include histograms.
CockroachDB does not support multi-column histograms.

If you are an advanced user and need to disable histogram collection for troubleshooting or performance tuning reasons, change the sql.stats.histogram_collection.enabled cluster setting by running SET CLUSTER SETTING as follows:

SET CLUSTER SETTING sql.stats.histogram_collection.enabled = false;

When sql.stats.histogram_collection.enabled is set to false, histograms are never collected, either as part of automatic statistics collection or by manually invoking CREATE STATISTICS.

Query plan cache

CockroachDB uses a cache for the query plans generated by the optimizer. This can lead to faster query execution since the database can reuse a query plan that was previously calculated, rather than computing a new plan each time a query is executed.

The query plan cache is enabled by default. To disable it, execute the following statement:

> SET CLUSTER SETTING sql.query_cache.enabled = false;

Only the following statements use the plan cache:

Join reordering

For a query involving multiple joins, the cost-based optimizer will explore additional join orderings in an attempt to find the lowest-cost execution plan, which can lead to significantly better performance in some cases.

Because this process leads to an exponential increase in the number of possible execution plans for such queries, it's only used to reorder subtrees containing 4 or fewer joins by default.

To change this setting, which is controlled by the reorder_joins_limit session variable, run the following statement. To disable this feature, set the variable to 0.

> SET reorder_joins_limit = 6;

Warning:

We strongly recommend not setting this value higher than 8 to avoid performance degradation. If set too high, the cost of generating and costing execution plans can end up dominating the total execution time of the query.

For more information about the difficulty of selecting an optimal join ordering, see our blog post An Introduction to Join Ordering.

Join hints

To force the use of a specific join algorithm even if the optimizer determines that a different plan would have a lower cost, you can use a join hint. You specify a join hint as <join type> <join algorithm> JOIN. For example:

INNER HASH JOIN
OUTER MERGE JOIN
LEFT LOOKUP JOIN
CROSS HASH JOIN
INNER INVERTED JOIN
LEFT INVERTED JOIN

Note:

Due to SQL's implicit AS syntax, you cannot specify a join hint with only the join algorithm keyword (e.g., MERGE). For example, a MERGE JOIN b will be interpreted as having an implicit AS and be executed as a AS MERGE JOIN b, which is equivalent to a JOIN b. Because the resulting query might execute without returning any hint-related error (because it is valid SQL), it will seem like the join hint "worked", but actually it didn't affect which join algorithm was used. The correct syntax is a INNER MERGE JOIN b.

For a join hint example, see Use the right join type.

Supported join algorithms

HASH: Forces a hash join; in other words, it disables merge and lookup joins. A hash join is always possible, even if there are no equality columns - CockroachDB considers the nested loop join with no index a degenerate case of the hash join (i.e., a hash table with one bucket).
MERGE: Forces a merge join, even if it requires re-sorting both sides of the join.
LOOKUP: Forces a lookup join into the right side; the right side must be a table with a suitable index. Note that LOOKUP can only be used with INNER and LEFT joins.
INVERTED: Forces an inverted join into the right side; the right side must be a table with a suitable GIN index. Note that INVERTED can only be used with INNER and LEFT joins.

Note:

You cannot use inverted joins on partial GIN indexes.

If it is not possible to use the algorithm specified in the hint, an error is signaled.

Note:

To make the optimizer prefer lookup joins to merge joins when performing foreign key checks, set the prefer_lookup_joins_for_fks session variable to on.

Additional considerations

This syntax is consistent with the SQL Server syntax for join hints, except that:
- SQL Server uses LOOP instead of LOOKUP.
- CockroachDB does not support LOOP and instead supports LOOKUP for the specific case of nested loop joins with an index.
When you specify a join hint, the two tables will not be reordered by the optimizer. The reordering behavior has the following characteristics, which can be affected by hints:
- Given a JOIN b, CockroachDB will not try to commute to b JOIN a. This means that you will need to pay attention to this ordering, which is especially important for lookup joins. Without a hint, a JOIN b might be executed as b INNER LOOKUP JOIN a using an index into a, whereas a INNER LOOKUP JOIN b requires an index into b.
- (a JOIN b) JOIN c might be changed to a JOIN (b JOIN c), but this does not happen if a JOIN b uses a hint; the hint forces that particular join to happen as written in the query.
You should reconsider hint usage with each new release of CockroachDB. Due to improvements in the optimizer, hints specified to work with an older version may cause decreased performance in a newer version.

Inverted join examples

To run these examples, initialize a demo cluster with the MovR workload.

Start the MovR database on a 3-node CockroachDB demo cluster with a larger data set.

cockroach demo movr --num-histories 250000 --num-promo-codes 250000 --num-rides 125000 --num-users 12500 --num-vehicles 3750 --nodes 3

Create a GIN index on the vehicles table's ext column.

CREATE INVERTED INDEX idx_vehicle_details ON vehicles(ext);

Check the statement plan for a SELECT statement that uses an inner inverted join.

EXPLAIN SELECT * FROM vehicles@primary AS v2 INNER INVERTED JOIN vehicles@idx_vehicle_details AS v1 ON v1.ext @> v2.ext;

                                           info
-------------------------------------------------------------------------------------------
  distribution: full
  vectorized: true

  • lookup join
  │ table: vehicles@primary
  │ equality: (city, id) = (city,id)
  │ equality cols are key
  │ pred: ext @> ext
  │
  └── • inverted join
      │ table: vehicles@idx_vehicle_details
      │
      └── • scan
            estimated row count: 3,750 (100% of the table; stats collected 3 minutes ago)
            table: vehicles@primary
            spans: FULL SCAN
(16 rows)

Time: 1ms total (execution 1ms / network 0ms)

You can omit the INNER INVERTED JOIN statement by putting v1.ext on the left side of a @> join condition in a WHERE clause and using an index hint for the GIN index.

EXPLAIN SELECT * FROM vehicles@idx_vehicle_details AS v1, vehicles AS v2 WHERE v1.ext @> v2.ext;

                                            info
--------------------------------------------------------------------------------------------
  distribution: full
  vectorized: true

  • lookup join
  │ table: vehicles@primary
  │ equality: (city, id) = (city,id)
  │ equality cols are key
  │ pred: ext @> ext
  │
  └── • inverted join
      │ table: vehicles@idx_vehicle_details
      │
      └── • scan
            estimated row count: 3,750 (100% of the table; stats collected 12 minutes ago)
            table: vehicles@primary
            spans: FULL SCAN
(16 rows)

Time: 1ms total (execution 1ms / network 0ms)

Use the LEFT INVERTED JOIN hint to perform a left inverted join.

EXPLAIN SELECT * FROM vehicles AS v2 LEFT INVERTED JOIN vehicles AS v1 ON v1.ext @> v2.ext;

                                            info
--------------------------------------------------------------------------------------------
  distribution: full
  vectorized: true

  • lookup join (left outer)
  │ table: vehicles@primary
  │ equality: (city, id) = (city,id)
  │ equality cols are key
  │ pred: ext @> ext
  │
  └── • inverted join (left outer)
      │ table: vehicles@idx_vehicle_details
      │
      └── • scan
            estimated row count: 3,750 (100% of the table; stats collected 16 minutes ago)
            table: vehicles@primary
            spans: FULL SCAN
(16 rows)

Time: 2ms total (execution 2ms / network 0ms)

Known Limitations

When a column is dropped from a multi-column index, the optimizer will not collect new statistics for the deleted column. However, the optimizer never deletes the old multi-column statistics. This can cause a buildup of statistics in system.table_statistics leading the optimizer to use stale statistics, which could result in sub-optimal plans. To workaround this issue and avoid these scenarios, explicitly delete those statistics from the system.table_statistics table.

Tracking GitHub Issue
Single-column statistics are not deleted when columns are dropped, which could cause minor performance issues.

Tracking GitHub Issue
The automatic statistics refresher automatically checks whether it needs to refresh statistics for every table in the database upon startup of each node in the cluster. If statistics for a table have not been refreshed in a while, this will trigger collection of statistics for that table. If statistics have been refreshed recently, it will not force a refresh. As a result, the automatic statistics refresher does not necessarily perform a refresh of statistics after an upgrade. This could cause a problem, for example, if the upgrade moves from a version without histograms to a version with histograms. To refresh statistics manually, use CREATE STATISTICS.

Tracking GitHub Issue

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Cost-Based Optimizer

How is cost calculated?

Table statistics

Control automatic statistics

Control statistics refresh rate

Turn off statistics

Control histogram collection

Query plan cache

Join reordering

Join hints

Supported join algorithms

Additional considerations

Inverted join examples

Known Limitations

See also