db: create simple hashtable of fields in SELECT.

This simplistically maps names to numbers, eg:

	SELECT foo, bar FROM tbl;

'foo' -> 0
'bar' -> 1

If a statement is too complex for our simple parsing, we treat it as a
single field (which currently it always is).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/sql-rewrite.py

@@ -83,13 +83,58 @@ rewriters = {
     "postgres": PostgresRewriter(),
 }
 
+
+# djb2 is simple and effective: see http://www.cse.yorku.ca/~oz/hash.html
+def hash_djb2(string):
+    val = 5381
+    for s in string:
+        val = ((val * 33) & 0xFFFFFFFF) ^ ord(s)
+    return val
+
+
+def colname_htable(query):
+    assert query.upper().startswith("SELECT")
+    colquery = query[6:query.upper().index(" FROM ")]
+    colnames = colquery.split(',')
+
+    # If split caused unbalanced brackets, it's complex: assume
+    # a single field!
+    if any([colname.count('(') != colname.count(')') for colname in colnames]):
+        return [('"' + colquery.strip() + '"', 0)]
+
+    # 50% density htable
+    tablesize = len(colnames) * 2 - 1
+    table = [("NULL", -1)] * tablesize
+    for colnum, colname in enumerate(colnames):
+        colname = colname.strip()
+        # SELECT xxx AS yyy -> Y
+        as_clause = colname.upper().find(" AS ")
+        if as_clause != -1:
+            colname = colname[as_clause + 4:].strip()
+
+        pos = hash_djb2(colname) % tablesize
+        while table[pos][0] != "NULL":
+            pos = (pos + 1) % tablesize
+        table[pos] = ('"' + colname + '"', colnum)
+    return table
+
+
 template = Template("""#ifndef LIGHTNINGD_WALLET_GEN_DB_${f.upper()}
 #define LIGHTNINGD_WALLET_GEN_DB_${f.upper()}
 
 #include <config.h>
+#include <ccan/array_size/array_size.h>
 #include <wallet/db_common.h>
 
 #if HAVE_${f.upper()}
+% for colname, table in colhtables.items():
+static const struct sqlname_map ${colname}[] = {
+% for t in table:
+    { ${t[0]}, ${t[1]} },
+% endfor
+};
+
+% endfor
 
 struct db_query db_${f}_queries[] = {
 
@@ -99,6 +144,10 @@ struct db_query db_${f}_queries[] = {
          .query = "${elem['query']}",
          .placeholders = ${elem['placeholders']},
          .readonly = ${elem['readonly']},
+% if elem['colnames'] is not None:
+         .colnames = ${elem['colnames']},
+         .num_colnames = ARRAY_SIZE(${elem['colnames']}),
+% endif
     },
 % endfor
 };
@@ -129,6 +178,7 @@ def extract_queries(pofile):
             if chunk != []:
                 yield chunk
 
+    colhtables = {}
     queries = []
     for c in chunk(pofile):
 
@@ -140,13 +190,21 @@ def extract_queries(pofile):
         # Strip header and surrounding quotes
         query = c[i][7:][:-1]
 
+        is_select = query.upper().startswith("SELECT")
+        if is_select:
+            colnames = 'col_table{}'.format(len(queries))
+            colhtables[colnames] = colname_htable(query)
+        else:
+            colnames = None
+
         queries.append({
             'name': query,
             'query': query,
             'placeholders': query.count('?'),
-            'readonly': "true" if query.upper().startswith("SELECT") else "false",
+            'readonly': "true" if is_select else "false",
+            'colnames': colnames,
         })
-    return queries
+    return colhtables, queries
 
 
 if __name__ == "__main__":
@@ -165,7 +223,7 @@ if __name__ == "__main__":
 
     rewriter = rewriters[dialect]
 
-    queries = extract_queries(sys.argv[1])
+    colhtables, queries = extract_queries(sys.argv[1])
     queries = rewriter.rewrite(queries)
 
-    print(template.render(f=dialect, queries=queries))
+    print(template.render(f=dialect, queries=queries, colhtables=colhtables))

wallet/db_common.h

@@ -49,6 +49,10 @@ struct db_query {
 	/* Is this a read-only query? If it is there's no need to tell plugins
 	 * about it. */
 	bool readonly;
+
+	/* If this is a select statement, what column names */
+	const struct sqlname_map *colnames;
+	size_t num_colnames;
 };
 
 enum db_binding_type {
@@ -155,5 +159,10 @@ AUTODATA_TYPE(db_backends, struct db_config);
  */
 void db_changes_add(struct db_stmt *db_stmt, const char * expanded);
 
+/* devtools/sql-rewrite.py generates this simple htable */
+struct sqlname_map {
+	const char *sqlname;
+	int val;
+};
 
 #endif /* LIGHTNING_WALLET_DB_COMMON_H */

wallet/wallet.c

@@ -4321,7 +4321,7 @@ struct amount_msat wallet_total_forward_fees(struct wallet *w)
 
 	stmt = db_prepare_v2(w->db, SQL("SELECT"
 					" CAST(COALESCE(SUM(in_msatoshi - out_msatoshi), 0) AS BIGINT)"
-					"FROM forwarded_payments "
+					" FROM forwarded_payments "
 					"WHERE state = ?;"));
 	db_bind_int(stmt, 0, wallet_forward_status_in_db(FORWARD_SETTLED));
 	db_query_prepared(stmt);