vsv: CSV files as virtual tables

Provides virtual table for working directly with CSV files, without importing data into the database. Useful for very large datasets.

Adapted from vsv.c by Keith Medcalf.

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Usage

.load ./vsv

create virtual table temp.vsv using vsv(...);
select * from vsv;

The parameters to the vsv module (the vsv(...) part) are as follows:

filename=STRING     the filename, passed to the Operating System
data=STRING         alternative data
schema=STRING       Alternate Schema to use
columns=N           columns parsed from the VSV file
header=BOOL         whether or not a header row is present
skip=N              number of leading data rows to skip
rsep=STRING         record separator
fsep=STRING         field separator
validatetext=BOOL   validate UTF-8 encoding of text fields
affinity=AFFINITY    affinity to apply to each returned value
nulls=BOOL          empty fields are returned as NULL

Defaults:

filename / data     nothing.  You must provide one or the other
                    it is an error to provide both or neither

schema              nothing.  If not provided then one will be
                    generated for you from the header, or if no
                    header is available then autogenerated using
                    field names manufactured as cX where X is the
                    column number

columns             nothing.  If not specified then the number of
                    columns is determined by counting the fields
                    in the first record of the VSV file (which
                    will be the header row if header is specified),
                    the number of columns is not parsed from the
                    schema even if one is provided

header=no           no header row in the VSV file
skip=0              do not skip any data rows in the VSV file
fsep=','            default field separator is a comma
rsep='\n'           default record separator is a newline
validatetext=no     do not validate text field encoding
affinity=none       do not apply affinity to each returned value
nulls=off           empty fields returned as zero-length

Parameter types:

STRING means a quoted string
N means a whole number not containing a sign
BOOL means something that evaluates as true or false. Case insensitive: yes, no, true, false, 1, 0. Defaults to true
AFFINITY means an SQLite3 type specification. Case insensitive: none, blob, text, integer, real, numeric
STRING means a quoted string. The quote character may be either a single quote or a double quote. Two quote characters in a row will be replaced with a single quote character. STRINGS do not need to be quoted if it is obvious where they begin and end (that is, they do not contain a comma). Leading and trailing spaces will be trimmed from unquoted strings.

The separator string containing exactly one character, or a valid escape sequence. Recognized escape sequences are:

\t horizontal tab, ascii character 9 (0x09)
\n linefeed, ascii character 10 (0x0a)
\v vertical tab, ascii character 11 (0x0b)
\f form feed, ascii character 12 (0x0c)
\xhh specific byte where hh is hexadecimal

The validatetext setting will cause the validity of the field encoding (not its contents) to be verified. It effects how fields that are supposed to contain text will be returned to the SQLite3 library in order to prevent invalid utf8 data from being stored or processed as if it were valid utf8 text.

The nulls option will cause fields that do not contain anything to return NULL rather than an empty result. Two separators side-by-each with no intervening characters at all will be returned as NULL if nulls is true and if nulls is false or the contents are explicity empty ("") then a 0 length blob (if affinity=blob) or 0 length text string.

For the affinity setting, the following processing is applied to each value returned by the VSV virtual table:

none no affinity is applied, all fields will be returned as text just like in the original csv module, embedded nulls will terminate the text. if validatetext is in effect then an error will be thrown if the field does not contain validly encoded text or contains embedded nulls
blob all fields will be returned as blobs validatetext has no effect
text all fields will be returned as text just like in the original csv module, embedded nulls will terminate the text. if validatetext is in effect then a blob will be returned if the field does not contain validly encoded text or the field contains embedded nulls
integer if the field data looks like an integer, (regex "^ _(+|-)?\d+ _$"), then an integer will be returned as provided by the compiler and platform runtime strtoll function otherwise the field will be processed as text as defined above
real if the field data looks like a number, (regex "^ (+|-)?(\d+.?\d|\d*.?\d+)(eE?\d+)? *$") then a double will be returned as provided by the compiler and platform runtime strtold function otherwise the field will be processed as text as defined above
numeric if the field looks like an integer (see integer above) that integer will be returned; if the field looks like a number (see real above) then the number will returned as an integer if it has no fractional part; otherwise a double will be returned

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