Reduce per-row allocations

[zolstein]

Modify several internal functions to avoid allocating unnecessary memory. This commit employs three strategies:

• Allocate a scans slice only when scanning the first row, and store it inside the RowScanner to reuse on subsequent rows.
• When scanning into slices, use more sophistocated reflection code to extend the slie while avoid temporary slice header allocations.
• When scanning into slices of non-pointers, scan directly into the slice index, rather than allocate a new value and copy after scanning.

---

I used a simple but (hopefully) vaguely representative benchmarking program to measure the impact.

Hyperfine shows an end-to-end runtime reduction of 40%.

# Without change
$ go build main.go && hyperfine --warmup=1 "./main -bench"           12:25:17
Benchmark 1: ./main -bench
  Time (mean ± σ):     15.984 s ±  0.115 s    [User: 93.516 s, System: 20.190 s]
  Range (min … max):   15.825 s … 16.258 s    10 runs
 
# With change
$ go build main.go && hyperfine --warmup=1 "./main -bench"              12:28:54
Benchmark 1: ./main -bench
  Time (mean ± σ):      9.186 s ±  0.094 s    [User: 60.400 s, System: 8.723 s]
  Range (min … max):    9.020 s …  9.385 s    10 runs

This also reduced allocated objects (when making 1000 queries returning 1000 rows each) by roughly 3 million and allocated space by ~150MB. (I can share the pprof SVGs if you want.)

Here's the (condensed) text: of the benchmark, if you want to replicate.

type Person struct {
	Name string
	Age  sql.Null[int64]
}

type Record struct {
	ID int64
	Person
	CreatedAt time.Time
}

func main() {
	ctx := context.Background()

	numThreads := 10
	numIters := 5000
	var wg sync.WaitGroup
	wg.Add(numThreads)
	for t := 0; t < numThreads; t++ {
		go func(t int) {
			_, err := runQueries(ctx, numIters)
			if err != nil {
				fmt.Fprintf(os.Stderr, "QueryRow failed: %v\n", err)
				os.Exit(1)
			}
			wg.Done()
		}(t)
	}
	wg.Wait()
}

func runQueries(ctx context.Context, numIters int) ([]Record, error) {
	conn, err := pgx.Connect(ctx, "postgres://zolstein:password@localhost:5432/db")
	if err != nil {
		fmt.Fprintf(os.Stderr, "Unable to connect to database: %v\n", err)
		os.Exit(1)
	}
	defer conn.Close(ctx)

	records := make([]Record, 1000)
	for i := 0; i < numIters; i++ {
		records = records[:0]
		rows, err := conn.Query(ctx, "select id, name, age, created_at from people") // Returns 1000 rows
		_ = rows
		if err != nil {
			return nil, err
		}
		err = pgxscan.ScanAll(&records, rows)
		if err != nil {
			return nil, err
		}
		_ = records
	}
	return records, nil
}

[codecov]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 80.72%. Comparing base (981a63a) to head (9e763fa).

Files	Patch %	Lines
dbscan/dbscan.go	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   80.41%   80.72%   +0.30%     
==========================================
  Files           5        5              
  Lines         531      555      +24     
==========================================
+ Hits          427      448      +21     
- Misses         89       92       +3     
  Partials       15       15              

Flag	Coverage Δ
unittests	80.72% <92.50%> (+0.30%)	⬆️

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[georgysavva]

Thank you for this PR. It looks very good!

[georgysavva]

@zolstein here is the new release with the change: https://github.com/georgysavva/scany/releases/tag/v2.1.1