Warn against pre-existing indexes.

Pre-existing indexes will reduce data loading performances and it's
generally better to DROP the index prior to the load and CREATE them
again once the load is done. See #251 for an example of that.

In that patch we just add a WARNING against the situation, the next
patch will also add support for a new WITH clause option allowing to
have pgloader take care of the DROP/CREATE dance around the data
loading.

src/package.lisp

@@ -178,6 +178,7 @@
 	   #:list-databases
 	   #:list-tables
 	   #:list-columns
+	   #:list-indexes
 	   #:list-tables-cols
 	   #:list-tables-and-fkeys
 	   #:list-reserved-keywords
@@ -244,6 +245,8 @@
   (:use #:cl
         #:pgloader.params #:pgloader.utils #:pgloader.connection
         #:pgloader.sources #:pgloader.queue)
+  (:import-from #:pgloader.pgsql
+                #:list-indexes)
   (:export #:*csv-path-root*
            #:csv-connection
            #:specs

src/pgsql/queries.lisp

@@ -189,6 +189,31 @@
      where c.oid = '~:[~*~a~;~a.~a~]'::regclass and attnum > 0
   order by attnum" schema schema table-name) :column)))
 
+(defun list-indexes (pgconn table-name)
+  "List all indexes for TABLE-NAME in SCHEMA."
+  (with-pgsql-connection (pgconn)
+    (loop :for (index-name table-name table-oid primary sql)
+       :in (pomo:query (format nil "
+select i.relname,
+       indrelid::regclass,
+       indrelid,
+       indisprimary,
+       pg_get_indexdef(indexrelid)
+  from pg_index x
+       join pg_class i ON i.oid = x.indexrelid
+ where indrelid = '~@[~a.~]~a'::regclass"
+                               (when (typep table-name 'cons)
+                                 (car table-name))
+                               (typecase table-name
+                                 (cons   (cdr table-name))
+                                 (string table-name))))
+       :collect (make-pgsql-index :name index-name
+                                  :table-name table-name
+                                  :table-oid table-oid
+                                  :primary primary
+                                  :columns nil
+                                  :sql sql))))
+
 (defun list-reserved-keywords (pgconn)
   "Connect to PostgreSQL DBNAME and fetch reserved keywords."
   (handler-case

src/pgsql/schema.lisp

@@ -263,7 +263,7 @@
 ;;;
 ;;; Index support
 ;;;
-(defstruct pgsql-index name table-name table-oid primary unique columns)
+(defstruct pgsql-index name table-name table-oid primary unique columns sql)
 
 (defgeneric index-table-name (index)
   (:documentation
@@ -293,18 +293,20 @@
        (values
         ;; ensure good concurrency here, don't take the ACCESS EXCLUSIVE
         ;; LOCK on the table before we have the index done already
-        (format nil "CREATE UNIQUE INDEX ~a ON ~a (~{~a~^, ~});"
-                index-name table-name cols)
+        (or (pgsql-index-sql index)
+            (format nil "CREATE UNIQUE INDEX ~a ON ~a (~{~a~^, ~});"
+                    index-name table-name cols))
         (format nil
                 "ALTER TABLE ~a ADD PRIMARY KEY USING INDEX ~a;"
                 table-name index-name)))
 
       (t
-       (format nil "CREATE~:[~; UNIQUE~] INDEX ~a ON ~a (~{~a~^, ~});"
-               (pgsql-index-unique index)
-               index-name
-               table-name
-               cols)))))
+       (or (pgsql-index-sql index)
+           (format nil "CREATE~:[~; UNIQUE~] INDEX ~a ON ~a (~{~a~^, ~});"
+                   (pgsql-index-unique index)
+                   index-name
+                   table-name
+                   cols))))))
 
 ;;;
 ;;; Parallel index building.

src/sources/csv/csv.lisp

@@ -179,7 +179,15 @@
 	 (*state*        (or *state* (pgloader.utils:make-pgstate)))
 	 (lp:*kernel*    (make-kernel 2))
 	 (channel        (lp:make-channel))
-	 (queue          (lq:make-queue :fixed-capacity *concurrent-batches*)))
+	 (queue          (lq:make-queue :fixed-capacity *concurrent-batches*))
+         (indexes        (list-indexes (target-db csv)
+                                       (target csv))))
+
+    ;; issue a performance warning against pre-existing indexes
+    (when indexes
+      (log-message :warning "Target table ~s has ~d indexes defined against it."
+                   (target csv) (length indexes))
+      (log-message :warning "That could impact loading performance badly"))
 
     (with-stats-collection ((target csv)
                             :dbname (db-name (target-db csv))