Sometimes it is useful to know how much does this data will take on
disk, with blockSerializedSize() you can know this (although without
compression).

This can be a major knowledge for various aggregation functions that
tracking some state (i.e. uniqCombined).

Change test '00305_http_and_readonly' because DROP TABLE IF EXISTS now works in readonly mode in case the table doesn't exist.

Accept String or FixedString data types as key for maps in the
SummingMergeTree now that sumMap supports these two types (introduced in
bc0fbd68).
Signed-off-by: NBaudouin Giard <bgiard@bloomberg.net>

Trying to resurrect https://github.com/ClickHouse/ClickHouse/pull/7533.
I'd like to get this PR in if we have an agreement on syntax and general
direction, after that I'll rebase actual alter functionality from above mentioned PR.

Adapt sumMap to accept an array of strings as the key column. This is useful when we do not want or cannot make the keys be numbers.
Signed-off-by: NBaudouin Giard <bgiard@bloomberg.net>

Before we reported LOGICAL_ERROR that is incorrect.