feat: Add microbenchmark for string functions

microbenchmarks/README.md

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+
+# Microbenchmarks
+
+This directory contains microbenchmarks for comparing DataFusion and DuckDB performance on individual SQL functions. Unlike the TPC-H and TPC-DS benchmarks which test full query execution, these microbenchmarks focus on the performance of specific SQL functions and expressions.
+
+## Overview
+
+The benchmarks generate synthetic data, write it to Parquet format, and then measure the execution time of various SQL functions across both DataFusion and DuckDB. Results include per-function timing comparisons and summary statistics.
+
+## Setup
+
+Create a virtual environment and install dependencies:
+
+```shell
+cd microbenchmarks
+python3 -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+```
+
+## Usage
+
+Run a benchmark:
+
+```shell
+python microbenchmarks.py
+```
+
+### Options
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `--rows` | `1000000` | Number of rows in the generated test data |
+| `--warmup` | `2` | Number of warmup iterations before timing |
+| `--iterations` | `5` | Number of timed iterations (results are averaged) |
+| `--output` | stdout | Output file path for markdown results |
+
+### Examples
+
+Run the benchmark with default settings:
+
+```shell
+python microbenchmark.py
+```
+
+Run the benchmark with 10 million rows:
+
+```shell
+python microbenchmarks.py --rows 10000000
+```
+
+Run the benchmark and save results to a file:
+
+```shell
+python microbenchmarks.py --output results.md
+```
+
+## Output
+
+The benchmark outputs a markdown table comparing execution times:
+
+| Function | DataFusion (ms) | DuckDB (ms) | Speedup | Faster |
+|----------|----------------:|------------:|--------:|--------|
+| trim | 12.34 | 15.67 | 1.27x | DataFusion |
+| lower | 8.90 | 7.50 | 1.19x | DuckDB |
+| ... | ... | ... | ... | ... |
+
+A summary section shows overall statistics including how many functions each engine won and total execution times.