Databases Reference
In-Depth Information
Then we initialize the
lookups
structure and call the magic function that will analyze
the
WHERE
clause and store the constants in this structure:
bzero((char*) &lookups, sizeof(lookups));
if (calc_lookup_values_from_cond(thd, cond,
tables, &lookups))
return 0;
The
calc_lookup_values_from_cond()
returns 1 when a field value is compared
to
NULL
. As our field cannot be
NULL
, we can return right away—nothing that we can
put in the table will satisfy the
WHERE
condition.
Otherwise we try to fill the table:
for (num = 0, ptr = output; *ptr; ptr++)
{
if (lookups.value1.str &&
my_strnncoll(cs, (const uchar*)*ptr, strlen(*ptr),
(const uchar*)lookups.value1.str,
lookups.value1.length))
continue;
If the
WHERE
clause has given us a value that we can filter on—that is, if the
str
pointer of the
lookups.value1
is not 0—we compare it with the word that we
want to put into the row. For comparison we use the
my_strnncoll()
function that
compares two strings using the collation, passed as the first argument (
system_
charset_info
is
utf8_general_ci
), and takes both strings and their lengths too
(indeed, the string is not necessarily zero terminated; after all, one can write
WHERE
field='a\0b'
).
If the value matches the word or if there is no value in the
lookups.value1
at all, we
store a new row in the table:
table->field[0]->store(++num);
table->field[1]->store(*ptr, strlen(*ptr), cs);
if (schema_table_store_record(thd, table))
return 1;
}
return 0;
}
