optimize_trivial_* session settings

These settings are available in system.settings and are autogenerated from source.

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optimize_trivial_approximate_count_query

Use an approximate value for trivial count optimization of storages that support such estimation, for example, EmbeddedRocksDB. Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

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optimize_trivial_count_query

Enables or disables the optimization to trivial query SELECT count() FROM table using metadata from MergeTree. If you need to use row-level security, disable this setting. Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

See also:

• optimize_functions_to_subcolumns

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optimize_trivial_count_with_sparsity_filter

Extends the optimize_trivial_count_query optimization to queries of the form SELECT count() FROM t WHERE col <op> const, where <op> const exactly partitions rows into defaults and non-defaults of col. The count is then served from the per-column num_defaults / num_rows counters that MergeTree already keeps in serialization.json, with no data scan. Patterns recognised:

• col = default(col) / col != default(col) for Int* / UInt*, String / FixedString, Date / DateTime / DateTime64, Decimal*, UUID, IPv4 / IPv6.
• IS NULL / IS NOT NULL on Nullable columns.
• empty(col) / notEmpty(col) on String columns.
• col = true / col != true on Bool columns.
• col > 0, col >= 1, col < 1, col <= 0 on unsigned integer columns.
• Bare col / NOT col on Int*, UInt*, Bool columns (truthy test).

The equality patterns are not applied to Float*, Enum*, Nullable, LowCardinality, or composite types (Tuple, Array, Map, …) — for these the count is served from the regular scan path. To take effect, the per-part num_defaults counter must be exact. Enable the MergeTree table setting compute_exact_num_defaults_for_sparse_columns on the target table before inserts and merges. Parts written without it are silently opted out of the rewrite, so enabling optimize_trivial_count_with_sparsity_filter alone is not enough. For the IS NULL / IS NOT NULL patterns on Nullable columns, the column must also have a num_defaults entry in serialization.json, which only happens when the MergeTree table setting nullable_serialization_version is set to allow_sparse at insert / merge time. With the default value basic Nullable columns get no per-column entry, so the optimization silently does not apply. Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

See also:

• optimize_trivial_count_query

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optimize_trivial_group_by_limit_query

Enables or disables the optimization of a trivial query SELECT key_expr FROM table GROUP BY key_expr LIMIT n (with no aggregate functions in the projection, no HAVING/ORDER BY/LIMIT BY/window clauses, and no GROUP BY modifiers) by setting max_rows_to_group_by = n + offset with group_by_overflow_mode = 'any'. The aggregation stops once n + offset distinct keys are produced. The optimization is suppressed when the user has explicitly set group_by_overflow_mode to a non-any value (to preserve their explicit throw/break contract), and when the user has already set a tighter max_rows_to_group_by (the optimization would be a no-op). Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

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optimize_trivial_insert_select

Optimize trivial ‘INSERT INTO table SELECT … FROM TABLES’ query