Inverted index: Replace setting density with max_rows_in_postings_list

[HarryLeeIBM]

The current implementation of creating inverted index uses index type inverted(tokenizer, density) in which density controls the size of postings lists for tokens. The original idea was to take the number of rows of the token exceeds the index_granularity * density as a judgement for the token's frequency. We found this setting is confusing to customers, and more importantly this turns out to be problematic since it causes lots of tokens skipping the postings list building and triggering full-scan during queries as a consequence since it is very easy to exceed this value when building posting lists.

The PR introduces a much more straight-forward way in which the second parameter of inverted(tokenizer, max_rows_per_postings_list) controls the maximum number of postings list. The following variants exist:

• inverted(ngrams, max_rows_per_postings_list): Use given max_rows_per_postings_list (assuming it is not 0)
• inverted(ngrams, 0): No limitation of maximum rows
• inverted(ngram): uses a default maximum rows which is 64K.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Inverted indexes do not store tokens with too many matches (i.e. row ids in the posting list). This saves space and avoids ineffective index lookups when sequential scans would be equally fast or faster. The previous heuristics (density parameter passed to the index definition) that controlled when tokens would not be stored was too confusing for users. A much simpler heuristics based on parameter max_rows_per_postings_list (default: 64k) is introduced which directly controls the maximum allowed number of row ids in a postings list.

[robot-clickhouse]

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