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Support negative indexes in split_part().
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This provides a handy way to get, say, the last field of the string.
Use of a negative index in this way has precedent in the nearby
left() and right() functions.

The implementation scans the string twice when N < -1, but it seems
likely that N = -1 will be the huge majority of actual use cases,
so I'm not really excited about adding complexity to avoid that.

Nikhil Benesch, reviewed by Jacob Champion; cosmetic tweakage by me

Discussion: https://postgr.es/m/cbb7f861-6162-3a51-9823-97bc3aa0b638@gmail.com
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@tglsfdc
tglsfdc committed Nov 13, 2020
1 parent 3bf4430 commit ec0294f
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Showing 4 changed files with 172 additions and 17 deletions.
8 changes: 7 additions & 1 deletion doc/src/sgml/func.sgml
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Original file line number Diff line number Diff line change
Expand Up @@ -3356,11 +3356,17 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
<para>
Splits <parameter>string</parameter> at occurrences
of <parameter>delimiter</parameter> and returns
the <parameter>n</parameter>'th field (counting from one).
the <parameter>n</parameter>'th field (counting from one),
or when <parameter>n</parameter> is negative, returns
the |<parameter>n</parameter>|'th-from-last field.
</para>
<para>
<literal>split_part('abc~@~def~@~ghi', '~@~', 2)</literal>
<returnvalue>def</returnvalue>
</para>
<para>
<literal>split_part('abc,def,ghi,jkl', ',', -2)</literal>
<returnvalue>ghi</returnvalue>
</para></entry>
</row>

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83 changes: 68 additions & 15 deletions src/backend/utils/adt/varlena.c
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Original file line number Diff line number Diff line change
Expand Up @@ -51,7 +51,7 @@ typedef struct varlena VarString;
typedef struct
{
bool is_multibyte; /* T if multibyte encoding */
bool is_multibyte_char_in_char;
bool is_multibyte_char_in_char; /* need to check char boundaries? */

char *str1; /* haystack string */
char *str2; /* needle string */
Expand Down Expand Up @@ -1439,8 +1439,7 @@ text_position_next_internal(char *start_ptr, TextPositionState *state)
/*
* Return a pointer to the current match.
*
* The returned pointer points into correct position in the original
* the haystack string.
* The returned pointer points into the original haystack string.
*/
static char *
text_position_get_match_ptr(TextPositionState *state)
Expand Down Expand Up @@ -1471,12 +1470,27 @@ text_position_get_match_pos(TextPositionState *state)
}
}

/*
* Reset search state to the initial state installed by text_position_setup.
*
* The next call to text_position_next will search from the beginning
* of the string.
*/
static void
text_position_reset(TextPositionState *state)
{
state->last_match = NULL;
state->refpoint = state->str1;
state->refpos = 0;
}

static void
text_position_cleanup(TextPositionState *state)
{
/* no cleanup needed */
}


static void
check_collation_set(Oid collid)
{
Expand Down Expand Up @@ -4581,9 +4595,8 @@ replace_text_regexp(text *src_text, void *regexp,

/*
* split_part
* parse input string
* return ord item (1 based)
* based on provided field separator
* parse input string based on provided field separator
* return N'th item (1 based, negative counts from end)
*/
Datum
split_part(PG_FUNCTION_ARGS)
Expand All @@ -4600,10 +4613,10 @@ split_part(PG_FUNCTION_ARGS)
bool found;

/* field number is 1 based */
if (fldnum < 1)
if (fldnum == 0)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("field position must be greater than zero")));
errmsg("field position must not be zero")));

inputstring_len = VARSIZE_ANY_EXHDR(inputstring);
fldsep_len = VARSIZE_ANY_EXHDR(fldsep);
Expand All @@ -4612,32 +4625,72 @@ split_part(PG_FUNCTION_ARGS)
if (inputstring_len < 1)
PG_RETURN_TEXT_P(cstring_to_text(""));

/* empty field separator */
/* handle empty field separator */
if (fldsep_len < 1)
{
/* if first field, return input string, else empty string */
if (fldnum == 1)
/* if first or last field, return input string, else empty string */
if (fldnum == 1 || fldnum == -1)
PG_RETURN_TEXT_P(inputstring);
else
PG_RETURN_TEXT_P(cstring_to_text(""));
}

/* find the first field separator */
text_position_setup(inputstring, fldsep, PG_GET_COLLATION(), &state);

/* identify bounds of first field */
start_ptr = VARDATA_ANY(inputstring);
found = text_position_next(&state);

/* special case if fldsep not found at all */
if (!found)
{
text_position_cleanup(&state);
/* if field 1 requested, return input string, else empty string */
if (fldnum == 1)
/* if first or last field, return input string, else empty string */
if (fldnum == 1 || fldnum == -1)
PG_RETURN_TEXT_P(inputstring);
else
PG_RETURN_TEXT_P(cstring_to_text(""));
}

/*
* take care of a negative field number (i.e. count from the right) by
* converting to a positive field number; we need total number of fields
*/
if (fldnum < 0)
{
/* we found a fldsep, so there are at least two fields */
int numfields = 2;

while (text_position_next(&state))
numfields++;

/* special case of last field does not require an extra pass */
if (fldnum == -1)
{
start_ptr = text_position_get_match_ptr(&state) + fldsep_len;
end_ptr = VARDATA_ANY(inputstring) + inputstring_len;
text_position_cleanup(&state);
PG_RETURN_TEXT_P(cstring_to_text_with_len(start_ptr,
end_ptr - start_ptr));
}

/* else, convert fldnum to positive notation */
fldnum += numfields + 1;

/* if nonexistent field, return empty string */
if (fldnum <= 0)
{
text_position_cleanup(&state);
PG_RETURN_TEXT_P(cstring_to_text(""));
}

/* reset to pointing at first match, but now with positive fldnum */
text_position_reset(&state);
found = text_position_next(&state);
Assert(found);
}

/* identify bounds of first field */
start_ptr = VARDATA_ANY(inputstring);
end_ptr = text_position_get_match_ptr(&state);

while (found && --fldnum > 0)
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74 changes: 73 additions & 1 deletion src/test/regress/expected/strings.out
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Original file line number Diff line number Diff line change
Expand Up @@ -1552,8 +1552,56 @@ SELECT replace('yabadoo', 'bad', '') AS "yaoo";
--
-- test split_part
--
select split_part('','@',1) AS "empty string";
empty string
--------------

(1 row)

select split_part('','@',-1) AS "empty string";
empty string
--------------

(1 row)

select split_part('joeuser@mydatabase','',1) AS "joeuser@mydatabase";
joeuser@mydatabase
--------------------
joeuser@mydatabase
(1 row)

select split_part('joeuser@mydatabase','',2) AS "empty string";
empty string
--------------

(1 row)

select split_part('joeuser@mydatabase','',-1) AS "joeuser@mydatabase";
joeuser@mydatabase
--------------------
joeuser@mydatabase
(1 row)

select split_part('joeuser@mydatabase','',-2) AS "empty string";
empty string
--------------

(1 row)

select split_part('joeuser@mydatabase','@',0) AS "an error";
ERROR: field position must be greater than zero
ERROR: field position must not be zero
select split_part('joeuser@mydatabase','@@',1) AS "joeuser@mydatabase";
joeuser@mydatabase
--------------------
joeuser@mydatabase
(1 row)

select split_part('joeuser@mydatabase','@@',2) AS "empty string";
empty string
--------------

(1 row)

select split_part('joeuser@mydatabase','@',1) AS "joeuser";
joeuser
---------
Expand All @@ -1578,6 +1626,30 @@ select split_part('@joeuser@mydatabase@','@',2) AS "joeuser";
joeuser
(1 row)

select split_part('joeuser@mydatabase','@',-1) AS "mydatabase";
mydatabase
------------
mydatabase
(1 row)

select split_part('joeuser@mydatabase','@',-2) AS "joeuser";
joeuser
---------
joeuser
(1 row)

select split_part('joeuser@mydatabase','@',-3) AS "empty string";
empty string
--------------

(1 row)

select split_part('@joeuser@mydatabase@','@',-2) AS "mydatabase";
mydatabase
------------
mydatabase
(1 row)

--
-- test to_hex
--
Expand Down
24 changes: 24 additions & 0 deletions src/test/regress/sql/strings.sql
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Original file line number Diff line number Diff line change
Expand Up @@ -533,8 +533,24 @@ SELECT replace('yabadoo', 'bad', '') AS "yaoo";
--
-- test split_part
--
select split_part('','@',1) AS "empty string";

select split_part('','@',-1) AS "empty string";

select split_part('joeuser@mydatabase','',1) AS "joeuser@mydatabase";

select split_part('joeuser@mydatabase','',2) AS "empty string";

select split_part('joeuser@mydatabase','',-1) AS "joeuser@mydatabase";

select split_part('joeuser@mydatabase','',-2) AS "empty string";

select split_part('joeuser@mydatabase','@',0) AS "an error";

select split_part('joeuser@mydatabase','@@',1) AS "joeuser@mydatabase";

select split_part('joeuser@mydatabase','@@',2) AS "empty string";

select split_part('joeuser@mydatabase','@',1) AS "joeuser";

select split_part('joeuser@mydatabase','@',2) AS "mydatabase";
Expand All @@ -543,6 +559,14 @@ select split_part('joeuser@mydatabase','@',3) AS "empty string";

select split_part('@joeuser@mydatabase@','@',2) AS "joeuser";

select split_part('joeuser@mydatabase','@',-1) AS "mydatabase";

select split_part('joeuser@mydatabase','@',-2) AS "joeuser";

select split_part('joeuser@mydatabase','@',-3) AS "empty string";

select split_part('@joeuser@mydatabase@','@',-2) AS "mydatabase";

--
-- test to_hex
--
Expand Down

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