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This is a performance bug, affecting in particular the bison generated code for the parser. Prior to this fix, the grammar used a long chain of reduces to parse an expression, like: bit_expr -> bit_term bit_term -> bit_factor bit_factor -> value_expr value_expr -> term term -> factor etc This chain of reduces cause the internal state automaton in the generated parser to execute more state transitions and more reduces, so that the generated MySQLParse() function would spend a lot of time looping to execute all the grammar reductions. With this patch, the grammar has been reorganized so that rules are more "flat", limiting the depth of reduces needed to parse <expr>. Tests have been written to enforce that relative priorities and properties of operators have not changed while changing the grammar. See the bug report for performance data. mysql-test/r/parser_precedence.result: Improved test coverage for operator precedence mysql-test/t/parser_precedence.test: Improved test coverage for operator precedence sql/sql_yacc.yy: Simplified the grammar to improve performances