executor: reuse chunk to reduce memory usage in union scan #7717
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What problem does this PR solve?
For a large transaction, union scan uses lots of memory which is harmful to the performance.
What is changed and how it works?
Reuse chunk in when build added rows.
Check List
Tests
execute 4900 update statements which affected 4900 rows in one transaction.
Code changes
Side effects
PTAL @coocood @lysu