To us, the word Item means more than just
“thingamabob”; it is a technical term with a precise
definition in the context of our source code.
Item is a class. Each instance of the
Item class has:
an analogue in the SQL language
a data type descriptor
All of the following SQL “thingamabobs” are modeled
session or global variables
SQL functions (not a surprise since SQL functions have data types and return values).
In the function category we include operators
operators are merely functions that return values. We also include
operators such as
LIKE, which are operators that return boolean
values. Consider the following statement:
SELECT UPPER(column1) FROM t WHERE column2 = @x;
For this statement, MySQL will need to store a list of items for the select list ('column1' column reference and UPPER function), and a list of items for the WHERE clause ('column2' column reference and '@x' variable and '=' operator).
Terminology: an Item instance in a MySQL program roughly corresponds to a "site", which according to the standard_SQL definition is "a place that holds an instance of a value of a specified data type", Another word that you'll see often in MySQL code is "field", which means column reference, and the Item_field subclass is generally for column values that occur for the intersection of a row and column in a table.
MySQL's Item class is defined in .../sql/item.h, and its subclasses are defined in .../sql/item*.h (that is, in item.h, item_cmpfunc.h, item_func.h, item_geofunc.h, item_row.h, item_strfunc.h, item_subselect.h, item_sum.h, item_timefunc.h). Page-width limitations prevent us from displaying the whole tree, but these are the main Item subclasses, and the subclasses of the subclasses:
Item_ident (Item_field, Item_ref) Item_null Item_num (Item_int, Item_real) Item_param Item_string (Item_static_string_func, Item_datetime, Item_empty_string) Item_hex_string (Item_bin_string) Item_result_field (all "item_func.h" "item_subselect.h" "item_sub.h" classes) Item_copy_string Item_cache (Item_cache_int, Item_cache_real, Item_cache_str, Item_cache_row) Item_type_holder Item_row
There's no formal classification of subclasses, but the main distinctions are by use (field, parameter, function) and by data type (num, string).
So, how does MySQL use items? You'll find that nearly every .cc program in the /sql directory makes some use of the Item class and its subclasses, so this list of programs is only partial and very general:
sql_parse.cc: Makes new items in add_field_to_list() item_sum.cc: Uses item_func subclasses for COUNT, AVG, SUM item_buff.cc: Where buffers for item values can be stored item_cmpfunc.cc: Comparison functions with item_func subclasses item_create.cch: For creating items that the lex might use item_subselect.cc: Subqueries are another type of function mysqld.cc: When main() ends, it uses clean_up() for items opt_range.cc: Uses field, compare-condition, and value subclasses procedure.cc: Notice Procedure * has a pointer to an item list protocol.cc: Uses send_fields() to pass item values back to users sys_var.cc: System variables have Item associations too sql_base.cc: Thread-specific Item searchers like find_field_in_table() sql_class.cc: Look at cleanup_after_query() sql_delete.cc: This (like sql_insert.cc etc.) has field references sql_error.cc: Has one of many examples of SHOW's use of items sql_lex.cc: Notice "add...to_list" functions sql_select.cc: The largest program that uses items, apparently udf_example.cc: The comments in this program are extensive
Whenever there's a need for an SQL operation that assigns, compares, aggregates, accepts, sends, or validates a site, you'll find a MySQL use of Item and its subclasses.