Enabling the Bison Trace

Add the following snippet to the C declarations section of src/backend/parser/gram.y:

#ifdef YYDEBUG
#undef YYDEBUG
#endif
#define YYDEBUG 1
int base_yydebug = 1;
int plpgsql_yydebug = 1;   /* matches %name-prefix used by pl_gram.y */
#define YYFPRINTF trace_parser

#include <stdio.h>
#include "utils/guc.h"

static void trace_parser(FILE *stream, const char *fmt, ...)
    __attribute__((format(printf, 2, 3)));

static void trace_parser(FILE *stream, const char *fmt, ...) {
    StringInfoData buf;
    va_list        ap;
    int            needed;
    char           path[1024];
    FILE          *fp;

    if (client_min_messages != LOG)
        return;

    initStringInfo(&buf);
    for (;;) {
        va_start(ap, fmt);
        needed = appendStringInfoVA(&buf, fmt, ap);
        va_end(ap);
        if (needed == 0)
            break;
        enlargeStringInfo(&buf, needed);
    }

    snprintf(path, sizeof(path), "%s/pgbison.trace", getenv("HOME"));
    fp = fopen(path, "a");
    if (!fp)
        elog(ERROR, "cannot open trace file %s", path);
    fwrite(buf.data, 1, buf.len, fp);
    fclose(fp);

    ereport(LOG, (errmsg_internal("'%s'", buf.data)));
    pfree(buf.data);
}

Connnect with psql and run:

SET client_min_messages = 'log';

Execute:

SELECT datetime '2024-01-04 20:16:00';

A trace file appears at $HOME/pgbison.trace.

Reading the Trace

Open the file from bottom to top and search backward for by rule.

1. parse_toplevel

   Reducing stack by rule 1 (line 1746):
      $1 = nterm stmtmulti (: )
   -> $$ = nterm parse_toplevel (: )
   

   Corresponds to:

   parse_toplevel: stmtmulti
   

2. smttmulti

   Reducing stack by rule 7 (line 1794):
      $1 = nterm stmtmulti (: )
      $2 = token ';' (: )
      $3 = nterm stmt (: )
   -> $$ = nterm stmtmulti (: )
   

   Rule:

   stmtmulti: stmtmulti ';' stmt
   

3. stmt

   Reducing stack by rule 137 (line 1945):
   -> $$ = nterm stmt (: )
   

   Empty rule.

4. stmtmulti (single statemant)

   Reducing stack by rule 8 (line 1806):
      $1 = nterm stmt (: )
   -> $$ = nterm stmtmulti (: )
   

   Rule:

   stmtmulti: stmt
   

5. stmt (SELECT)

   Reducing stack by rule 127 (line 1934):
      $1 = nterm SelectStmt (: )
   -> $$ = nterm stmt (: )
   

   Rule:

   stmt: SelectStmt
   

6. SelectStmt

   Reducing stack by rule 2211 (line 17578):
      $1 = nterm select_no_parens (: )
   -> $$ = nterm SelectStmt (: )
   

   Rule:

   SelectStmt: select_no_parens
   

7. select_no_parens

   Reducing stack by rule 2215 (line 17599):
      $1 = nterm simple_select (: )
   -> $$ = nterm select_no_parens (: )
   

   Rule:

   select_no_parens: simple_select
   

8. simple_select

   Reducing stack by rule 2225 (line 17686):
      $1 = token SELECT (: )
      $2 = nterm hint_string (: )
      $3 = nterm opt_all_clause (: )
      $4 = nterm opt_target_list (: )
      $5 = nterm into_clause (: )
      $6 = nterm from_clause (: )
      $7 = nterm where_clause (: )
      $8 = nterm hierarchical_query_clause (: )
      $9 = nterm group_having_clause (: )
      $10 = nterm window_clause (: )
   -> $$ = nterm simple_select (: )
   

9. Optional clauses (all empty)

   Reducing stack by rule 2780 (line 21511): -> nterm window_clause
   Reducing stack by rule 2118 (line 16717): -> nterm group_having_clause
   Reducing stack by rule 2238 (line 17826): -> nterm hierarchical_query_clause
   Reducing stack by rule 2428 (line 19025): -> nterm where_clause
   Reducing stack by rule 2341 (line 18401): -> nterm from_clause
   Reducing stack by rule 2257 (line 17940): -> nterm into_clause
   

10. opt_target_list

    Reducing stack by rule 2914 (line 22182):
       $1 = nterm target_list (: )
    -> $$ = nterm opt_target_list (: )
    

    target_list

    Reducing stack by rule 2916 (line 22187):
       $1 = nterm target_el (: )
    -> $$ = nterm target_list (: )
    

11. target_el

    Reducing stack by rule 2920 (line 22236):
       $1 = nterm a_expr (: )
    -> $$ = nterm target_el (: )
    

    a_expr

    Reducing stack by rule 2563 (line 19974):
       $1 = nterm c_expr (: )
    -> $$ = nterm a_expr (: )
    

    c_expr

    Reducing stack by rule 2664 (line 20515):
       $1 = nterm AexprConst (: )
    -> $$ = nterm c_expr (: )
    

12. AexprConst

    Reducing stack by rule 2959 (line 22717):
       $1 = nterm func_name (: )
       $2 = nterm Sconst (: )
    -> $$ = nterm AexprConst (: )
    

13. Sconst

    Reducing stack by rule 2969 (line 22797):
       $1 = token SCONST (: )
    -> $$ = nterm Sconst (: )
    

    The literal '2024-01-04 20:16:00'.

14. func_name

    Reducing stack by rule 2950 (line 22665):
       $1 = nterm type_function_name (: )
    -> $$ = nterm func_name (: )
    

    type_function_name

    Reducing stack by rule 2989 (line 22968):
       $1 = nterm unreserved_keyword (: )
    -> $$ = nterm type_function_name (: )
    

15. unreserved_keyword

    Reducing stack by rule 3071 (line 23095):
       $1 = token DATETIME (: )
    -> $$ = nterm unreserved_keyword (: )
    

16. Empty rules

    Reducing stack by rule 2275 (line 18021): -> nterm opt_all_clause
    Reducing stack by rule 2235 (line 17794): -> nterm hint_string
    

All components of simple_select are now complete.