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

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merge_selector_algorithm

The algorithm to select parts for merges assignment

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merge_selector_base

Affects write amplification of assigned merges (expert level setting, don’t change if you don’t understand what it is doing). Works for Simple and StochasticSimple merge selectors

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merge_selector_blurry_base_scale_factor

Controls when the logic kicks in relatively to the number of parts in partition. The bigger the factor the more belated reaction will be.

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merge_selector_enable_heuristic_to_lower_max_parts_to_merge_at_once

Enable heuristic for simple merge selector which will lower maximum limit for merge choice. By doing so number of concurrent merges will increase which can help with TOO_MANY_PARTS errors but at the same time this will increase the write amplification.

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merge_selector_enable_heuristic_to_remove_small_parts_at_right

Enable heuristic for selecting parts for merge which removes parts from right side of range, if their size is less than specified ratio (0.01) of sum_size. Works for Simple and StochasticSimple merge selectors

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merge_selector_heuristic_to_lower_max_parts_to_merge_at_once_exponent

Controls the exponent value used in formulae building lowering curve. Lowering exponent will lower merge widths which will trigger increase in write amplification. The reverse is also true.

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merge_selector_window_size

How many parts to look at once.
Last modified on July 24, 2026