79 lines
2.3 KiB
Markdown
79 lines
2.3 KiB
Markdown
# sqlite-parquet-vtable
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A SQLite [virtual table](https://sqlite.org/vtab.html) extension to expose Parquet files as SQL tables.
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## Caveats
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I'm not an experienced C/C++ programmer. This library is definitely not bombproof. It's good enough for my use case,
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and may be good enough for yours, too.
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* I don't use `sqlite3_malloc` and `sqlite3_free` for C++ objects
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* Maybe this doesn't matter, since portability isn't a goal
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* The C -> C++ interop definitely leaks some C++ exceptions
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* Obvious cases like file not found and unsupported Parquet types are OK
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* Low memory conditions aren't handled gracefully.
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## Building
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1. Install [`parquet-cpp`](https://github.com/apache/parquet-cpp)
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1. Master appears to be broken for text row group stats; see https://github.com/cldellow/sqlite-parquet-vtable/issues/5 for which versions to use
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2. Run `./build-sqlite` to fetch and build the SQLite dev bits
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3. Run `./parquet/make` to build the module
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1. You will need to fixup the paths in this file to point at your local parquet-cpp folder.
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## Use
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```
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$ sqlite/sqlite3
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sqlite> .load parquet/libparquet
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sqlite> CREATE VIRTUAL TABLE demo USING parquet('parquet-generator/100-rows-1.parquet');
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sqlite> SELECT * FROM demo;
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...if all goes well, you'll see data here!...
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```
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## Supported features
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### Row group filtering
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Row group filtering is supported for strings and numerics so long as the SQLite
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type matches the Parquet type.
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e.g. if you have a column `foo` that is an INT32, this query will skip row groups whose
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statistics prove that it does not contain relevant rows:
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```
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SELECT * FROM tbl WHERE foo = 123;
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```
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but this query will devolve to a table scan:
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```
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SELECT * FROM tbl WHERE foo = '123';
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```
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This is laziness on my part and could be fixed without too much effort.
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### Row filtering
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For common constraints, the row is checked to see if it satisfies the query's
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constraints before returning control to SQLite's virtual machine. This minimizes
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the number of allocations performed when many rows are filtered out by
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the user's criteria.
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### Types
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These Parquet types are supported:
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* INT96 timestamps (exposed as milliseconds since the epoch)
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* INT8/INT16/INT32/INT64
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* UTF8 strings
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* BOOLEAN
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* FLOAT
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* DOUBLE
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* Variable- and fixed-length byte arrays
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These are not currently supported:
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* UINT8/UINT16/UINT32/UINT64
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* DECIMAL
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