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This event takes place just before ddl_command_end, and is fired if and only if at least one object has been dropped by the command. (For instance, DROP TABLE IF EXISTS of a table that does not in fact exist will not lead to such a trigger firing). Commands that drop multiple objects (such as DROP SCHEMA or DROP OWNED BY) will cause a single event to fire. Some firings might be surprising, such as ALTER TABLE DROP COLUMN. The trigger is fired after the drop has taken place, because that has been deemed the safest design, to avoid exposing possibly-inconsistent internal state (system catalogs as well as current transaction) to the user function code. This means that careful tracking of object identification is required during the object removal phase. Like other currently existing events, there is support for tag filtering. To support the new event, add a new pg_event_trigger_dropped_objects() set-returning function, which returns a set of rows comprising the objects affected by the command. This is to be used within the user function code, and is mostly modelled after the recently introduced pg_identify_object() function. Catalog version bumped due to the new function. Dimitri Fontaine and Álvaro Herrera Review by Robert Haas, Tom Lane
src/backend/parser/README Parser ====== This directory does more than tokenize and parse SQL queries. It also creates Query structures for the various complex queries that are passed to the optimizer and then executor. parser.c things start here scan.l break query into tokens scansup.c handle escapes in input strings kwlookup.c turn keywords into specific tokens keywords.c table of standard keywords (passed to kwlookup.c) gram.y parse the tokens and produce a "raw" parse tree analyze.c top level of parse analysis for optimizable queries parse_agg.c handle aggregates, like SUM(col1), AVG(col2), ... parse_clause.c handle clauses like WHERE, ORDER BY, GROUP BY, ... parse_coerce.c handle coercing expressions to different data types parse_collate.c assign collation information in completed expressions parse_cte.c handle Common Table Expressions (WITH clauses) parse_expr.c handle expressions like col, col + 3, x = 3 or x = 4 parse_func.c handle functions, table.column and column identifiers parse_node.c create nodes for various structures parse_oper.c handle operators in expressions parse_param.c handle Params (for the cases used in the core backend) parse_relation.c support routines for tables and column handling parse_target.c handle the result list of the query parse_type.c support routines for data type handling parse_utilcmd.c parse analysis for utility commands (done at execution time)