Outputs

Synth writes files — it never opens a network or DB connection.

synth.WriteCSV("users.csv", users)
synth.WriteJSONL("users.jsonl", users)
synth.WriteSQL("users.sql", "users", users) // INSERT statements you run yourself

synth.Stream[User](100_000_000).ToCSV("users.csv") // constant memory

CSV, JSONL, SQL INSERT files, Parquet, Postgres COPY files, and Debezium-shaped CDC events — every one of them a file. Handing it to your loader is the last step, and it is yours.

Postgres COPY

For bulk loading, COPY is what Postgres wants — an INSERT per row is the slowest path the server offers:

synth gen -s users.yaml -n 100000000 -o users.pgbin   # binary COPY
synth gen -s users.yaml -n 100000000 -o users.pgcopy  # text COPY

Both write a matching CREATE TABLE next to the data as users.pgbin.sql:

psql -f users.pgbin.sql

That pairing is not a convenience. Binary COPY carries no type names, so the table's column types are what the server decodes the bytes as, and a table built by hand that differs in one column means a rejected file. Synth generates the DDL and the encoding from the same type table, so they cannot disagree.

Parquet

A first-class output — pick it by extension or -f parquet:

synth gen -s users.yaml -n 100000 -o users.parquet
synth gen --preset user -n 50 -f parquet -o users.parquet

The same writer is available from Go:

parquet.WriteStructs("users.parquet", users)              // from Go structs
parquet.WriteRows("users.parquet", spec.Columns(), rows)  // from YAML/DDL/profiling

Column types are inferred (int64, double, boolean, string), so query engines see real types rather than everything-as-string. Uploading the file to S3, MinIO or a warehouse is your loader's job.

Parquet needs a real path

A Parquet file carries a footer, so it does not stream to stdout or through the gzip/zstd sink, and --append does not apply to it.

Compression

Large outputs compress on the way out — name the file and Synth does the rest:

synth gen -s users.yaml -n 100000000 -o users.jsonl.zst
synth gen -s users.yaml -n 100000000 -o users.csv.gz

Format summary

Format	Pick it with	Streams	--append
CSV	.csv, -f csv	yes	yes
JSONL	.jsonl, -f jsonl	yes	yes
SQL INSERT	.sql, -f sql	yes	yes
Postgres text COPY	.pgcopy, -f pgcopy	yes	no
Postgres binary COPY	.pgbin, -f pgcopy-binary	yes	no
Parquet	.parquet, -f parquet	no	no
CDC events	synth cdc	yes	—