PS-10232 [8.0]: Fix performance regression by limiting rows_in_buffer to query LIMIT
#5774
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…r` to query `LIMIT` Issue: `rows_in_buffer` could exceed the number of rows requested by a `SELECT ... LIMIT` query. This could lead to unnecessary memory usage and inefficient fetching. Fix: Adjust `rows_in_buffer` to respect the `SELECT` query's `LIMIT`. Now, `rows_in_buffer` is capped at the query limit, aligning buffer allocation with the expected number of rows.
dinodork
reviewed
Dec 4, 2025
| ulonglong select_limit = | ||
| std::max<ulonglong>(q_block->select_limit->val_uint(), 2); | ||
| rows_in_buffer = std::min(rows_in_buffer, select_limit); | ||
| } |
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The solution works, and I notice a speedup of about a factor three. I don't see any serious problems with it. It's not ideal that the executor peeks into the AST tree, however. My take on the coding pattern is that this type of information should be supplied by the iterator. This is probably something they would pick on if we proposed the patch upstream.
Before the regression, set_record_buffer() would cap the number of rows in the buffer at max_rows. So a more conservative approach would be to reintroduce that cap instead of peeking at the LIMIT, I guess?
What do you think?
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Issue:
rows_in_buffercould exceed the number of rows requested by aSELECT ... LIMITquery. This could lead to unnecessary memory usage and inefficient fetching.Fix: Adjust
rows_in_bufferto respect theSELECTquery'sLIMIT. Now,rows_in_bufferis capped at the query limit, aligning buffer allocation with the expected number of rows.