Use list.extend() instead of += in PrestoQuery.execute()

[ilonapapava]

Summary

PrestoQuery.execute() concatenates fetched pages using +=:

self._result._rows += self.fetch()

This creates a new list on every iteration, briefly holding 2x the data in memory (the old list + the new combined list) before the old one is garbage collected.

Replacing with .extend() appends in-place, avoiding the intermediate copy.

Benchmark

Tested with 50K-row result sets (~30 columns each) in a production org sync workload:

Metric	Before (+=)	After (.extend())	Improvement
VmPeak	4,149 MB	2,826 MB	-32%
RSS during processing	~2,700 MB	~1,600 MB	-40%

Changes

One line: += → .extend() in prestodb/client.py line 550.

[linux-foundation-easycla]

• ❌ - login: @ilonapapava / name: Ilona Papava . The commit (50042da) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Replaces list concatenation with in-place extension in PrestoQuery.execute() to avoid repeated list reallocations and substantially reduce peak and steady-state memory usage when fetching query results.

Flow diagram for updated PrestoQuery.execute loop using list.extend()

flowchart TD
    A[PrestoQuery_execute] --> B{_finished or _cancelled}
    B -- no --> C[call fetch]
    C --> D[rows = fetch_result]
    D --> E["_result._rows.extend(rows)"]
    E --> B
    B -- yes --> F[return _result]

Loading

File-Level Changes

Change	Details	Files
Optimize result row accumulation in PrestoQuery.execute() to avoid creating a new list on each fetch iteration.	Replace list concatenation using "+=" with in-place list extension using .extend(...) when appending fetched rows to the result. Preserve existing query execution flow and control conditions (_finished/_cancelled) while improving memory efficiency of row accumulation.	prestodb/client.py

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