[pgpool-hackers: 4333] Allow to load balance PREPARE

[Tatsuo Ishii]

Currently it is not possible load balance PREPARE even if the prepared
query is read only SELECT. For example,

PREPARE foo AS SELECT 1;

is always sent to primary node (in streaming replication mode or
logical replication mode), or main node (in other mode).

Attached patch is trying to load balance PREPARE (and following
EXECUTE/DEALLOCATE). Documentation changes are not included yet.

For this following changes are made:

- is_select_query() looks into "query" member if node is
  PrepareStmt. Also second argument "sql" (query string) is now
  mandatory. If sql is NULL, warning is emitted and this function
  returns false. If allow_sql_comments is off and node is PrepareStmt
  and query is SelectStmt, is_select_query() does not return false
  anymore.

- pool_has_function_call() looks into "query" member if node is
  PrepareStmt.

- Add PREPARE/EXECUTE/DEALLOCATE test cases to 001.load_balance test.

- Add send_prepare() function which is similar to parse_before_bind in
  extended query protocol case to keep up
  disable_load_balance_on_write rule. send_prepare() is called by
  SimpleQuery() when EXECUTE message is sent by frontend in SL mode so
  that it sends PREPARE message to primary node if it has not sent to
  primary because of load balance.  Note that send_prepare() does
  nothing if the clustering mode is other than SL mode. In native
  replication mode or snapshot isolation mode, all backend has the
  same data, and there's no point to keep up
  disable_load_balance_on_write rule.

Comments/suggestions are welcome.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp
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