Reading from the query cache is implemented using processor
"SourceFromChunks" which inherits from ISource. The latter has logic
which finishes processing (= calls .finish() on the output port +
returns Status::Finished) when the derived class returns an empty chunk.
If this empty chunk is not the last chunk, i.e. if it is followed by
non-empty chunks, the query result will be incorrect. This situation
should theoretically never occur in practice but who knows ... To be on
the safe side, writing into the query cache now rejects empty chunks and
thereby avoids this scenario.
After #36425 there was a lot of confusions/problems with configuring pools - when the message was confusing, and settings need to be ajusted in several places.
See some examples in #44251, #43351, #47900, #46515.
The commit includes the following changes:
1) Introduced a unified mechanism for reading pool sizes from the configuration file(s). Previously, pool sizes were read from the Context.cpp with fallbacks to profiles, whereas main_config_reloader in Server.cpp read them directly without fallbacks.
2) Corrected the data type for background_merges_mutations_concurrency_ratio. It should be float instead of int.
3) Refactored the default values for settings. Previously, they were defined in multiple places throughout the codebase, but they are now defined in one place (or two, to be exact: Settings.h and ServerSettings.h).
4) Improved documentation, including the correct message in system.settings.
Additionally make the code more conform with #46550.
ClickHouse reads table data in blocks of 'max_block_size' rows. Due to
filtering, aggregation, etc., result blocks are typically much smaller
than 'max_block_size' but there are also cases where they are much
bigger. Setting 'query_cache_squash_partial_results' (enabled by
default) now controls if result blocks are squashed (if they are tiny)
or split (if they are large) into blocks of 'max_block_size' size before
insertion into the query result cache. This reduces performance of
writes into the query cache but improves compressability of cache
entries and provides more natural block granularity when query results
are later served from the query cache.
Entries in the query cache are now also compressed by default. This
reduces the overall memory consumption at the cost of slower writes into
/ reads from the query cache. To disable compression, use setting
'query_cache_compress_entries'.
