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In the previous coding, it was possible for a relation to be created via CREATE TABLE, CREATE VIEW, CREATE SEQUENCE, CREATE FOREIGN TABLE, etc. in a schema while that schema was meanwhile being concurrently dropped. This led to a pg_class entry with an invalid relnamespace value. The same problem could occur if a relation was moved using ALTER .. SET SCHEMA while the target schema was being concurrently dropped. This patch prevents both of those scenarios by locking the schema to which the relation is being added using AccessShareLock, which conflicts with the AccessExclusiveLock taken by DROP. As a desirable side effect, this also prevents the use of CREATE OR REPLACE VIEW to queue for an AccessExclusiveLock on a relation on which you have no rights: that will now fail immediately with a permissions error, before trying to obtain a lock. We need similar protection for all other object types, but as everything other than relations uses a slightly different set of code paths, I'm leaving that for a separate commit. Original complaint (as far as I could find) about CREATE by Nikhil Sontakke; risk for ALTER .. SET SCHEMA pointed out by Tom Lane; further details by Dan Farina; patch by me; review by Hitoshi Harada.
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)