Simplified a bit and more commented patch version is in attach.
Algorithm were switched to linear probing, it makes code simpler and
clearer.
Flag usages were toggled: now it indicates that hash table were built,
it also makes code clearer.
XidInXip and some other functions got comments.
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With regards,
Sokolov Yura aka funny_falcon
Postgres Professional: https://postgrespro.ru
The Russian Postgres Company