Shrink blocks in HashJoin on consuming half of maximum memory

[vdimir]

Changelog category (leave one):

• Improvement

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

• HashJoin tries to shrink internal buffers consuming half of maximal available memory (set by max_bytes_in_join)

Followup #50112 (comment)

---

SET max_bytes_in_join = '2G';

SELECT UserID, h1.Referer, h1.URL, h2.URL
FROM datasets.hits_v1 AS h1
INNER JOIN datasets.hits_v1 AS h2 ON h1.UserID = h2.UserID AND h1.URL = h2.Referer
WHERE h1.URL != '' AND h2.URL != '' AND h1.Referer != '' AND h2.Referer != ''
ORDER BY UserID ASC
LIMIT 1

<Debug> HashJoin: Shrinking stored blocks table after 954.20 MiB of 1.86 GiB consumed
<Debug> HashJoin: Shrinked stored blocks 193.78 MiB freed, new memory consumption is 760.42 MiB
...
<Debug> MemoryTracker: Peak memory usage (for query): 1.03 GiB.

While with SET max_bytes_in_join = '20G':

<Debug> MemoryTracker: Peak memory usage (for query): 1.24 GiB.

[robot-ch-test-poll2]

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[alexey-milovidov]

But it will work only if the limit for JOIN size is defined. Let's also take max_memory_usage into account. Also, let's use the current memory usage for the query.

[alexey-milovidov]

For comparison, see how we do it for external GROUP BY.

[vdimir]

How to combine this two metrics?
We can compare memory tracker delta with max_memory_usage and current_bytes_in_join with max_bytes_in_join separately (as I did in 0d51fc7).
Or compare max(memory_tracker_delta, current_bytes_in_join) and min(max_memory_usage, max_bytes_in_join).

[alexey-milovidov]

This will be good:

memory_tracker_delta >= 0.5 * max_memory_usage
|| current_bytes_in_join >= 0.5 * max_bytes_in_join