pgsql: Add string_to_table() function.

Add string_to_table() function.

This splits a string at occurrences of a delimiter. It is exactly like
string_to_array() except for producing a set of values instead of an
array of values. Thus, the relationship of these two functions is
the same as between regexp_split_to_table() and regexp_split_to_array().

Although the same results could be had from unnest(string_to_array()),
this is somewhat faster than that, and anyway it seems reasonable to
have it for symmetry with the regexp functions.

Pavel Stehule, reviewed by Peter Smith

Discussion: https://postgr.es/m/CAFj8pRD8HOpjq2TqeTBhSo_QkzjLOhXzGCpKJ4nCs7Y9SQkuPw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/66f163068030b5c5fe792a0daee27822dac43791

Modified Files
--------------
doc/src/sgml/func.sgml | 90 ++++++++++-----
src/backend/utils/adt/varlena.c | 214 +++++++++++++++++++++++++----------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 18 ++-
src/test/regress/expected/arrays.out | 108 ++++++++++++++++++
src/test/regress/sql/arrays.sql | 15 +++
6 files changed, 354 insertions(+), 93 deletions(-)