David Roberts 9d55c45b5a [ML] Improve multiline_start_pattern for CSV in find_file_structure (#51737)
The work to switch file upload over to treating delimited files
like semi-structured text and using the ingest pipeline for CSV
parsing makes the multi-line start pattern used for delimited
files much more critical than it used to be.

Previously it was always based on the time field, even if that
was towards the end of the columns, and no multi-line pattern
was created if no timestamp was detected.

This change improves the multi-line start pattern by:

1. Never creating a multi-line pattern if the sample contained
   only single line records.  This improves the import
   efficiency in a common case.
2. Choosing the leftmost field that has a well-defined pattern,
   whether that be the time field or a boolean/numeric field.
   This reduces the risk of a field with newlines occurring
   earlier, and also means the algorithm doesn't automatically
   fail for data without a timestamp.
2020-02-04 12:37:48 +00:00
..
2020-01-27 16:56:31 -05:00