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to eliminate unnecessary deadlocks. This commit adds SELECT ... FOR SHARE paralleling SELECT ... FOR UPDATE. The implementation uses a new SLRU data structure (managed much like pg_subtrans) to represent multiple- transaction-ID sets. When more than one transaction is holding a shared lock on a particular row, we create a MultiXactId representing that set of transactions and store its ID in the row's XMAX. This scheme allows an effectively unlimited number of row locks, just as we did before, while not costing any extra overhead except when a shared lock actually has to be shared. Still TODO: use the regular lock manager to control the grant order when multiple backends are waiting for a row lock. Alvaro Herrera and Tom Lane.
This directory does more than tokenize and parse SQL queries. It also creates Query structures for the various complex queries that is passed to the optimizer and then executor. parser.c things start here scan.l break query into tokens scansup.c handle escapes in input keywords.c turn keywords into specific tokens gram.y parse the tokens and fill query-type-specific structures analyze.c handle post-parse processing for each query type parse_clause.c handle clauses like WHERE, ORDER BY, GROUP BY, ... parse_coerce.c used for coercing expressions of different types parse_expr.c handle expressions like col, col + 3, x = 3 or x = 4 parse_oper.c handle operations in expressions parse_agg.c handle aggregates, like SUM(col1), AVG(col2), ... parse_func.c handle functions, table.column and column identifiers parse_node.c create nodes for various structures parse_target.c handle the result list of the query parse_relation.c support routines for tables and column handling parse_type.c support routines for type handling