114 lines
2.7 KiB
Markdown
114 lines
2.7 KiB
Markdown
# sqlite-parquet-vtable
|
|
|
|
A SQLite [virtual table](https://sqlite.org/vtab.html) extension to expose Parquet files as SQL tables. You may also find [csv2parquet](https://github.com/cldellow/csv2parquet/) useful.
|
|
|
|
This [blog post](https://cldellow.com/2018/06/22/sqlite-parquet-vtable.html) provides some context on why you might use this.
|
|
|
|
## Installing
|
|
|
|
### Download
|
|
|
|
You can fetch a version built for Ubuntu 16.04 at https://s3.amazonaws.com/cldellow/public/libparquet/libparquet.so.xz
|
|
|
|
### Building
|
|
|
|
```
|
|
./make-linux
|
|
```
|
|
|
|
The first run will git clone a bunch of libraries, patch them to be statically linkable and build them.
|
|
|
|
Subsequent builds will only build the parquet virtual table extension.
|
|
|
|
#### Tests
|
|
|
|
Run:
|
|
|
|
```
|
|
tests/create-queries-from-templates
|
|
tests/test-all
|
|
```
|
|
|
|
|
|
## Use
|
|
|
|
```
|
|
$ sqlite/sqlite3
|
|
sqlite> .load build/linux/libparquet
|
|
sqlite> CREATE VIRTUAL TABLE demo USING parquet('parquet-generator/99-rows-1.parquet');
|
|
sqlite> SELECT * FROM demo;
|
|
...if all goes well, you'll see data here!...
|
|
```
|
|
|
|
Note: if you get an error like:
|
|
|
|
```
|
|
sqlite> .load build/linux/libparquet
|
|
Error: parquet/libparquet.so: wrong ELF class: ELFCLASS64
|
|
```
|
|
|
|
You have the 32-bit SQLite installed. To fix this, do:
|
|
|
|
```
|
|
sudo apt-get remove --purge sqlite3
|
|
sudo apt-get install sqlite3:amd64
|
|
```
|
|
|
|
## Supported features
|
|
|
|
### Row group filtering
|
|
|
|
Row group filtering is supported for strings and numerics so long as the SQLite
|
|
type matches the Parquet type.
|
|
|
|
e.g. if you have a column `foo` that is an INT32, this query will skip row groups whose
|
|
statistics prove that it does not contain relevant rows:
|
|
|
|
```
|
|
SELECT * FROM tbl WHERE foo = 123;
|
|
```
|
|
|
|
but this query will devolve to a table scan:
|
|
|
|
```
|
|
SELECT * FROM tbl WHERE foo = '123';
|
|
```
|
|
|
|
This is laziness on my part and could be fixed without too much effort.
|
|
|
|
### Row filtering
|
|
|
|
For common constraints, the row is checked to see if it satisfies the query's
|
|
constraints before returning control to SQLite's virtual machine. This minimizes
|
|
the number of allocations performed when many rows are filtered out by
|
|
the user's criteria.
|
|
|
|
### Memoized slices
|
|
|
|
Individual clauses are mapped to the row groups they match.
|
|
|
|
eg going on row group statistics, which store minimum and maximum values, a clause
|
|
like `WHERE city = 'Dawson Creek'` may match 80% of row groups.
|
|
|
|
In reality, it may only be present in one or two row groups.
|
|
|
|
This is recorded in a shadow table so future queries that contain that clause
|
|
can read only the necessary row groups.
|
|
|
|
### Types
|
|
|
|
These Parquet types are supported:
|
|
|
|
* INT96 timestamps (exposed as milliseconds since the epoch)
|
|
* INT8/INT16/INT32/INT64
|
|
* UTF8 strings
|
|
* BOOLEAN
|
|
* FLOAT
|
|
* DOUBLE
|
|
* Variable- and fixed-length byte arrays
|
|
|
|
These are not currently supported:
|
|
|
|
* UINT8/UINT16/UINT32/UINT64
|
|
* DECIMAL
|