Handle possibly duplicated query

While quite unlikely, it's possible that powa_statements_src returns the same
entry pg_stat_statements entry but with duplicated query text, as reported by
github user gjedeer.

To fix it, simply store one of the possible many query text as all should be
semantically equivalent.  It's also possible that a duplicated entry is due to
a hash collision, happening after an entry is discarded by pg_stat_statements
on the remote database, but it seems so unlikely that this possibility is
entirely ignored.

Thanks to github user gjedeer for helping to diagnose this problem which has
existed for a very long time!

powa--4.1.2--4.1.3.sql

@@ -204,3 +204,89 @@ BEGIN
 END;
 $_$
 language plpgsql;
+
+CREATE OR REPLACE FUNCTION powa_statements_snapshot(_srvid integer) RETURNS void AS $PROC$
+DECLARE
+    result boolean;
+    v_funcname    text := 'powa_statements_snapshot';
+    v_rowcount    bigint;
+BEGIN
+    -- In this function, we capture statements, and also aggregate counters by database
+    -- so that the first screens of powa stay reactive even though there may be thousands
+    -- of different statements
+    -- We only capture databases that are still there
+    PERFORM powa_log(format('running %I', v_funcname));
+
+    PERFORM powa_prevent_concurrent_snapshot(_srvid);
+
+    WITH capture AS(
+        SELECT *
+        FROM powa_statements_src(_srvid)
+    ),
+    mru as (UPDATE powa_statements set last_present_ts = now()
+            FROM capture
+            WHERE powa_statements.queryid = capture.queryid
+              AND powa_statements.dbid = capture.dbid
+              AND powa_statements.userid = capture.userid
+              AND powa_statements.srvid = _srvid
+    ),
+    missing_statements AS(
+        INSERT INTO public.powa_statements (srvid, queryid, dbid, userid, query)
+            SELECT _srvid, queryid, dbid, userid, min(query)
+            FROM capture c
+            WHERE NOT EXISTS (SELECT 1
+                              FROM powa_statements ps
+                              WHERE ps.queryid = c.queryid
+                              AND ps.dbid = c.dbid
+                              AND ps.userid = c.userid
+                              AND ps.srvid = _srvid
+            )
+            GROUP BY queryid, dbid, userid
+    ),
+
+    by_query AS (
+        INSERT INTO public.powa_statements_history_current
+            SELECT _srvid, queryid, dbid, userid,
+            ROW(
+                ts, calls, total_exec_time, rows,
+                shared_blks_hit, shared_blks_read, shared_blks_dirtied,
+                shared_blks_written, local_blks_hit, local_blks_read,
+                local_blks_dirtied, local_blks_written, temp_blks_read,
+                temp_blks_written, blk_read_time, blk_write_time,
+                plans, total_plan_time,
+                wal_records, wal_fpi, wal_bytes
+            )::powa_statements_history_record AS record
+            FROM capture
+    ),
+
+    by_database AS (
+        INSERT INTO public.powa_statements_history_current_db
+            SELECT _srvid, dbid,
+            ROW(
+                ts, sum(calls),
+                sum(total_exec_time), sum(rows), sum(shared_blks_hit),
+                sum(shared_blks_read), sum(shared_blks_dirtied),
+                sum(shared_blks_written), sum(local_blks_hit),
+                sum(local_blks_read), sum(local_blks_dirtied),
+                sum(local_blks_written), sum(temp_blks_read),
+                sum(temp_blks_written), sum(blk_read_time), sum(blk_write_time),
+                sum(plans), sum(total_plan_time),
+                sum(wal_records), sum(wal_fpi), sum(wal_bytes)
+            )::powa_statements_history_record AS record
+            FROM capture
+            GROUP BY dbid, ts
+    )
+
+    SELECT count(*) INTO v_rowcount
+    FROM capture;
+
+    perform powa_log(format('%I - rowcount: %s',
+            v_funcname, v_rowcount));
+
+    IF (_srvid != 0) THEN
+        DELETE FROM powa_statements_src_tmp WHERE srvid = _srvid;
+    END IF;
+
+    result := true; -- For now we don't care. What could we do on error except crash anyway?
+END;
+$PROC$ language plpgsql; /* end of powa_statements_snapshot */

powa--4.1.3.sql

@@ -2536,7 +2536,7 @@ BEGIN
     ),
     missing_statements AS(
         INSERT INTO public.powa_statements (srvid, queryid, dbid, userid, query)
-            SELECT DISTINCT _srvid, queryid, dbid, userid, query
+            SELECT _srvid, queryid, dbid, userid, min(query)
             FROM capture c
             WHERE NOT EXISTS (SELECT 1
                               FROM powa_statements ps
@@ -2545,6 +2545,7 @@ BEGIN
                               AND ps.userid = c.userid
                               AND ps.srvid = _srvid
             )
+            GROUP BY queryid, dbid, userid
     ),
 
     by_query AS (