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BigQuery schema generator

Project description

# BigQuery Schema Generator

This script generates the BigQuery schema from the newline-delimited JSON data
records on the STDIN. The BigQuery data importer (`bq load`) uses only the
first 100 lines when the schema auto-detection feature is enabled. In contrast,
this script uses all data records to generate the schema.

$ generate-schema < > file.schema.json

Version: 0.3.1 (2019-01-18)

## Background

Data can be imported into [BigQuery]( using
the [bq]( command line
tool. It accepts a number of data formats including CSV or newline-delimited
JSON. The data can be loaded into an existing table or a new table can be
created during the loading process. The structure of the table is defined by
its [schema]( The table's
schema can be defined manually or the schema can be

When the auto-detect feature is used, the BigQuery data importer examines only
the first 100 records of the input data. In many cases, this is sufficient
because the data records were dumped from another database and the exact schema
of the source table was known. However, for data extracted from a service
(e.g. using a REST API) the record fields could have been organically added
at later dates. In this case, the first 100 records do not contain fields which
are present in later records. The **bq load** auto-detection fails and the data
fails to load.

The **bq load** tool does not support the ability to process the entire dataset
to determine a more accurate schema. This script fills in that gap. It
processes the entire dataset given in the STDIN and outputs the BigQuery schema
in JSON format on the STDOUT. This schema file can be fed back into the **bq
load** tool to create a table that is more compatible with the data fields in
the input dataset.

## Installation

Install from [PyPI]( repository using `pip3`.
If you want to install the package for your entire system globally, use
$ sudo -H pip3 install bigquery_schema_generator
If you are using a virtual environment (such as
[venv](, then you don't need
the `sudo` coommand, and you can just type:
$ pip3 install bigquery_schema_generator

A successful install should print out the following:
Collecting bigquery-schema-generator
Installing collected packages: bigquery-schema-generator
Successfully installed bigquery-schema-generator-0.1.4

The shell script `generate-schema` is installed in the same directory as

### Ubuntu Linux

Under Ubuntu Linux, you should find the `generate-schema` script at

### MacOS

If you installed Python from
[Python Releases for Mac OS X](,
then `/usr/local/bin/pip3` is a symlink to
`/Library/Frameworks/Python.framework/Versions/3.6/bin/pip3`. So
`generate-schema` is installed at

The Python installer updates `$HOME/.bash_profile` to add
`/Library/Frameworks/Python.framework/Versions/3.6/bin` to the `$PATH`
environment variable. So you should be able to run the `generate-schema`
command without typing in the full path.

## Usage

The `` script accepts a newline-delimited JSON data file on
the STDIN. (CSV is not supported currently.) It scans every record in the
input data file to deduce the table's schema. It prints the JSON formatted
schema file on the STDOUT. There are at least 3 ways to run this script:

**1) Shell script**

If you installed using `pip3`, then it should have installed a small helper
script named `generate-schema` in your local `./bin` directory of your current
environment (depending on whether you are using a virtual environment).

$ generate-schema < > file.schema.json

**2) Python module**

You can invoke the module directly using:
$ python3 -m bigquery_schema_generator.generate_schema < > file.schema.json
This is essentially what the `generate-schema` command does.

**3) Python script**

If you retrieved this code from its
[GitHub repository](,
then you can invoke the Python script directly:
$ ./ < > file.schema.json

### Schema Output

The resulting schema file can be given to the **bq load** command using the
`--schema` flag:

$ bq load --source_format NEWLINE_DELIMITED_JSON \
--ignore_unknown_values \
--schema file.schema.json \
mydataset.mytable \
where `mydataset.mytable` is the target table in BigQuery.

For debugging purposes, here is the equivalent `bq load` command using schema

$ bq load --source_format NEWLINE_DELIMITED_JSON \
--ignore_unknown_values \
--autodetect \
mydataset.mytable \

A useful flag for `bq load` is `--ignore_unknown_values`, which causes `bq
load` to ignore fields in the input data which are not defined in the schema.
When `` detects an inconsistency in the definition of a
particular field in the input data, it removes the field from the schema
definition. Without the `--ignore_unknown_values`, the `bq load` fails when
the inconsistent data record is read. Another useful flag during development and
debugging is `--replace` which replaces any existing BigQuery table.

After the BigQuery table is loaded, the schema can be retrieved using:

$ bq show --schema mydataset.mytable | python -m json.tool

(The `python -m json.tool` command will pretty-print the JSON formatted schema
file.) This schema file should be identical to `file.schema.json`.

### Flag Options

The `` script supports a handful of command line flags:

* `--help` Prints the usage with the list of supported flags.
* `--keep_nulls` Print the schema for null values, empty arrays or empty records.
* `--debugging_interval lines` Number of lines between heartbeat debugging messages. Default 1000.
* `--debugging_map` Print the metadata schema map for debugging purposes

#### Help (`--help`)

Print the built-in help strings:

$ generate-schema --help
usage: [-h] [--keep_nulls]
[--debugging_interval DEBUGGING_INTERVAL]

Generate BigQuery schema.

optional arguments:
-h, --help show this help message and exit
--keep_nulls Print the schema for null values, empty arrays or
empty records.
--debugging_interval DEBUGGING_INTERVAL
Number of lines between heartbeat debugging messages.
--debugging_map Print the metadata schema_map instead of the schema
for debugging

#### Keep Nulls (`--keep_nulls`)

Normally when the input data file contains a field which has a null, empty
array or empty record as its value, the field is suppressed in the schema file.
This flag enables this field to be included in the schema file.

In other words, using a data file containing just nulls and empty values:
$ generate_schema
{ "s": null, "a": [], "m": {} }
INFO:root:Processed 1 lines

With the `keep_nulls` flag, we get:
$ generate-schema --keep_nulls
{ "s": null, "a": [], "m": {} }
INFO:root:Processed 1 lines
"mode": "REPEATED",
"type": "STRING",
"name": "a"
"mode": "NULLABLE",
"fields": [
"mode": "NULLABLE",
"type": "STRING",
"name": "__unknown__"
"type": "RECORD",
"name": "d"
"mode": "NULLABLE",
"type": "STRING",
"name": "s"

#### Debugging Interval (`--debugging_interval`)

By default, the `` script prints a short progress message
every 1000 lines of input data. This interval can be changed using the
`--debugging_interval` flag.

$ generate-schema --debugging_interval 50 < > file.schema.json

#### Debugging Map (`--debugging_map`)

Instead of printing out the BigQuery schema, the `--debugging_map` prints out
the bookkeeping metadata map which is used internally to keep track of the
various fields and their types that were inferred using the data file. This
flag is intended to be used for debugging.

$ generate-schema --debugging_map < > file.schema.json

## Schema Types

### Supported Types

The **bq show --schema** command produces a JSON schema file that uses the
older [Legacy SQL date types](
For compatibility, **generate-schema** script will also generate a schema file
using the legacy data types.

The supported types are:

* `DATE`
* `TIME`

The `generate-schema` script supports both `NULLABLE` and `REPEATED` modes of
all of the above types.

The supported format of `TIMESTAMP` is as close as practical to the
[bq load format](
YYYY-[M]M-[D]D[( |T)[H]H:[M]M:[S]S[.DDDDDD]][time zone]
which appears to be an extension of the
[ISO 8601 format](
The difference from `bq load` is that the `[time zone]` component can be only
* `Z`
* `UTC` (same as `Z`)
* `(+|-)H[H][:M[M]]`

The suffix `UTC` is not standard ISO 8601 nor
[documented by Google](
but the `UTC` suffix is used by `bq extract` and the web interface. (See
[Issue 19](

Timezone names from the [tz database]( (e.g.
"America/Los_Angeles") are _not_ supported by `generate-schema`.

The following types are _not_ supported at all:

* `DATETIME` (unable to distinguish from `TIMESTAMP`)

### Type Inferrence Rules

The `generate-schema` script attempts to emulate the various type conversion and
compatibility rules implemented by **bq load**:

* `INTEGER` can upgrade to `FLOAT`
* if a field in an early record is an `INTEGER`, but a subsequent record
shows this field to have a `FLOAT` value, the type of the field will be
upgraded to a `FLOAT`
* the reverse does not happen, once a field is a `FLOAT`, it will remain a
* conflicting `TIME`, `DATE`, `TIMESTAMP` types upgrades to `STRING`
* if a field is determined to have one type of "time" in one record, then
subsequently a different "time" type, then the field will be assigned a
`STRING` type
* a field may be defined as `RECORD` (aka "Struct") type with `{ ... }`
* if the field is subsequently read as an array with a `[{ ... }]`, the
field is upgraded to a `REPEATED RECORD`
* a primitive type (`FLOAT`, `INTEGER`, `STRING`) cannot upgrade to a `REPEATED`
primitive type
* there's no technical reason why this cannot be allowed, but **bq load**
does not support it, so we follow its behavior
* a `DATETIME` field is always inferred to be a `TIMESTAMP`
* the format of these two fields is identical (in the absence of timezone)
* we follow the same logic as **bq load** and always infer these as
* `BOOLEAN`, `INTEGER`, and `FLOAT` can appear inside quoted strings
* In other words, `"true"` (or `"True"` or `"false"`, etc) is considered a
BOOLEAN type, `"1"` is considered an INTEGER type, and `"2.1"` is considered
a FLOAT type. Luigi Mori (jtschichold@) added additional logic to replicate
the type conversion logic used by `bq load` for these strings.
* `INTEGER` values overflowing a 64-bit signed integer upgrade to `FLOAT`
* integers greater than `2^63-1` (9223372036854775807)
* integers less than `-2^63` (-9223372036854775808)
* (See [Issue #18]( for more details)

## Examples

Here is an example of a single JSON data record on the STDIN (the `^D` below
means typing Control-D, which indicates "end of file" under Linux and MacOS):

$ generate-schema
{ "s": "string", "b": true, "i": 1, "x": 3.1, "t": "2017-05-22T17:10:00-07:00" }
INFO:root:Processed 1 lines
"mode": "NULLABLE",
"name": "b",
"type": "BOOLEAN"
"mode": "NULLABLE",
"name": "i",
"type": "INTEGER"
"mode": "NULLABLE",
"name": "s",
"type": "STRING"
"mode": "NULLABLE",
"name": "t",
"type": "TIMESTAMP"
"mode": "NULLABLE",
"name": "x",
"type": "FLOAT"

In most cases, the data file will be stored in a file:
$ cat >
{ "a": [1, 2] }
{ "i": 3 }

$ generate-schema < > file.schema.json
INFO:root:Processed 2 lines

$ cat file.schema.json
"mode": "REPEATED",
"name": "a",
"type": "INTEGER"
"mode": "NULLABLE",
"name": "i",
"type": "INTEGER"

## Benchmarks

I wrote the `bigquery_schema_generator/` script to create an
anonymized data file `tests/testdata/`:
$ ./bigquery_schema_generator/ < \
$ gzip
This data file is 290MB (5.6MB compressed) with 103080 data records.

Generating the schema using
$ bigquery_schema_generator/ < \
> anon1.schema.json
took 77s on a Dell Precision M4700 laptop with an Intel Core i7-3840QM CPU @
2.80GHz, 32GB of RAM, Ubuntu Linux 17.10, Python 3.6.3.

## System Requirements

This project was initially developed on Ubuntu 17.04 using Python 3.5.3. I have
tested it on:

* Ubuntu 18.04, Python 3.6.7
* Ubuntu 17.10, Python 3.6.3
* Ubuntu 17.04, Python 3.5.3
* Ubuntu 16.04, Python 3.5.2
* MacOS 10.14.2, [Python 3.6.4](
* MacOS 10.13.2, [Python 3.6.4](

## Changelog

See [](

## Authors

* Created by Brian T. Park (
* Additional type inferrence logic by Luigi Mori (jtschichold@).

## License

Apache License 2.0

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