R Internals: Data Sizes With Nullable Columns

Niels Berglund digs into the Binary Exchange Langage (BXL) and notices something weird about data sizes:

When looking at the data sent, the size of the packages and “drilling” into the TCP packets we could deduct that: :

  • Each column has an over-head of 32 bytes (at least for non nullable data)

  • The size of the column in one row is the size of the data type for numeric types.

  • For decimal and numeric an extra byte is added to each column, where this byte indicates the precision.

  • Columns of alpha numeric type all had 2 bytes pre-pended to the bytes, except max types.

  • For char and nchar the storage size was 2 bytes plus the size the column was defined as.

  • For varchar and nvarchar the storage size was 2 bytes plus the size of the data stored.

  • For the varmax data types the number of bytes that were pre-pended varied dependent on the data size.

Read the whole thing.

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