add cache of pool statements per query

refactor/reuse connection statement cache

spec/database_spec.cr

@@ -47,4 +47,20 @@ describe DB::Database do
       db.prepare("query3").should be_a(DB::PoolStatement)
     end
   end
+
+  it "should return same statement in pool per query" do
+    with_dummy do |db|
+      stmt = db.prepare("query1")
+      db.prepare("query2").should_not eq(stmt)
+      db.prepare("query1").should eq(stmt)
+    end
+  end
+
+  it "should close pool statements when closing db" do
+    stmt = uninitialized DB::PoolStatement
+    with_dummy do |db|
+      stmt = db.prepare("query1")
+    end
+    stmt.closed?.should be_true
+  end
 end

src/db.cr

@@ -119,6 +119,7 @@ module DB
 end
 
 require "./db/pool"
+require "./db/string_key_cache"
 require "./db/query_methods"
 require "./db/disposable"
 require "./db/database"

src/db/connection.cr

@@ -20,25 +20,20 @@ module DB
 
     # :nodoc:
     getter database
-    @statements_cache = {} of String => Statement
+    @statements_cache = StringKeyCache(Statement).new
 
     def initialize(@database : Database)
     end
 
     # :nodoc:
     def prepare(query) : Statement
-      stmt = @statements_cache.fetch(query, nil)
-      stmt = @statements_cache[query] = build_statement(query) unless stmt
-
-      stmt
+      @statements_cache.fetch(query) { build_statement(query) }
     end
 
     abstract def build_statement(query) : Statement
 
     protected def do_close
-      @statements_cache.each do |_, stmt|
-        stmt.close
-      end
+      @statements_cache.each_value &.close
       @statements_cache.clear
     end
   end

src/db/database.cr

@@ -24,6 +24,7 @@ module DB
 
     @pool : Pool(Connection)
     @setup_connection : Connection -> Nil
+    @statements_cache = StringKeyCache(PoolStatement).new
 
     # :nodoc:
     def initialize(@driver : Driver, @uri : URI)
@@ -48,14 +49,15 @@ module DB
 
     # Closes all connection to the database.
     def close
+      @statements_cache.each_value &.close
+      @statements_cache.clear
+
       @pool.close
     end
 
     # :nodoc:
     def prepare(query)
-      # TODO query based cache for pool statement
-      # TODO clear PoolStatements when closing the DB
-      PoolStatement.new self, query
+      @statements_cache.fetch(query) { PoolStatement.new(self, query) }
     end
 
     # :nodoc:

src/db/string_key_cache.cr

@@ -0,0 +1,21 @@
+module DB
+  class StringKeyCache(T)
+    @cache = {} of String => T
+
+    def fetch(key : String) : T
+      value = @cache.fetch(key, nil)
+      value = @cache[key] = yield unless value
+      value
+    end
+
+    def each_value
+      @cache.each do |_, value|
+        yield value
+      end
+    end
+
+    def clear
+      @cache.clear
+    end
+  end
+end