Support IF EXISTS/IF NOT EXISTS for DDL constructs

Added parameters :paramref:`_ddl.CreateTable.if_not_exists`,
:paramref:`_ddl.CreateIndex.if_not_exists`,
:paramref:`_ddl.DropTable.if_exists` and
:paramref:`_ddl.DropIndex.if_exists` to the :class:`_ddl.CreateTable`,
:class:`_ddl.DropTable`, :class:`_ddl.CreateIndex` and
:class:`_ddl.DropIndex` constructs which result in "IF NOT EXISTS" / "IF
EXISTS" DDL being added to the CREATE/DROP. These phrases are not accepted
by all databases and the operation will fail on a database that does not
support it as there is no similarly compatible fallback within the scope of
a single DDL statement.  Pull request courtesy Ramon Williams.

Fixes: #2843
Closes: #5663
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5663
Pull-request-sha: 748b8472345d96efb446e2a444fbe020b313669f

Change-Id: I6a2b1f697993ed49c31584f0a31887fb0a868ed3

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index bf257ab3f..5292b3901 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -2322,6 +2322,9 @@ class PGDDLCompiler(compiler.DDLCompiler):
             if concurrently:
                 text += "CONCURRENTLY "
 
+        if create.if_not_exists:
+            text += "IF NOT EXISTS "
+
         text += "%s ON %s " % (
             self._prepared_index_name(index, include_schema=False),
             preparer.format_table(index.table),
@@ -2405,6 +2408,9 @@ class PGDDLCompiler(compiler.DDLCompiler):
             if concurrently:
                 text += "CONCURRENTLY "
 
+        if drop.if_exists:
+            text += "IF EXISTS "
+
         text += self._prepared_index_name(index, include_schema=True)
         return text