Extend topic indexes for keyset pagination

The "keyset"-style pagination that Tildes uses for topic listings uses
WHERE and ORDER BY clauses that involve multiple columns to keep a
deterministic ordering even when the values in the main sort column are
equal. For example, when sorting by number of votes, you're actually
ordering by num_votes DESC, topic_id DESC. The previous single-column
indexes were a little inefficient for this and couldn't always be used
well.

This commit extends all of the relevant indexes to composite ones that
contain topic_id as well, and drops all of the original ones. This
should be more efficient, and should probably be done to indexes on the
comments table as well.

tildes/alembic/versions/f4e1ef359307_extend_topic_indexes_for_keyset_.py

@@ -0,0 +1,76 @@
+"""Extend topic indexes for keyset pagination
+
+Revision ID: f4e1ef359307
+Revises: 4e101aae77cd
+Create Date: 2020-01-15 20:52:23.380355
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "f4e1ef359307"
+down_revision = "4e101aae77cd"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_index(
+        "ix_topics_created_time_keyset",
+        "topics",
+        [sa.text("created_time DESC"), sa.text("topic_id DESC")],
+        unique=False,
+    )
+    op.create_index(
+        "ix_topics_last_activity_time_keyset",
+        "topics",
+        [sa.text("last_activity_time DESC"), sa.text("topic_id DESC")],
+        unique=False,
+    )
+    op.create_index(
+        "ix_topics_last_interesting_activity_time_keyset",
+        "topics",
+        [sa.text("last_interesting_activity_time DESC"), sa.text("topic_id DESC")],
+        unique=False,
+    )
+    op.create_index(
+        "ix_topics_num_comments_keyset",
+        "topics",
+        [sa.text("num_comments DESC"), sa.text("topic_id DESC")],
+        unique=False,
+    )
+    op.create_index(
+        "ix_topics_num_votes_keyset",
+        "topics",
+        [sa.text("num_votes DESC"), sa.text("topic_id DESC")],
+        unique=False,
+    )
+    op.drop_index("ix_topics_created_time", table_name="topics")
+    op.drop_index("ix_topics_last_activity_time", table_name="topics")
+    op.drop_index("ix_topics_last_interesting_activity_time", table_name="topics")
+    op.drop_index("ix_topics_num_comments", table_name="topics")
+    op.drop_index("ix_topics_num_votes", table_name="topics")
+
+
+def downgrade():
+    op.create_index("ix_topics_num_votes", "topics", ["num_votes"], unique=False)
+    op.create_index("ix_topics_num_comments", "topics", ["num_comments"], unique=False)
+    op.create_index(
+        "ix_topics_last_interesting_activity_time",
+        "topics",
+        ["last_interesting_activity_time"],
+        unique=False,
+    )
+    op.create_index(
+        "ix_topics_last_activity_time", "topics", ["last_activity_time"], unique=False
+    )
+    op.create_index("ix_topics_created_time", "topics", ["created_time"], unique=False)
+    op.drop_index("ix_topics_num_votes_keyset", table_name="topics")
+    op.drop_index("ix_topics_num_comments_keyset", table_name="topics")
+    op.drop_index(
+        "ix_topics_last_interesting_activity_time_keyset", table_name="topics"
+    )
+    op.drop_index("ix_topics_last_activity_time_keyset", table_name="topics")
+    op.drop_index("ix_topics_created_time_keyset", table_name="topics")

tildes/tildes/models/topic/topic.py

@@ -23,7 +23,7 @@ from sqlalchemy import (
 from sqlalchemy.dialects.postgresql import ENUM, JSONB, TSVECTOR
 from sqlalchemy.ext.mutable import MutableDict
 from sqlalchemy.orm import deferred, relationship
-from sqlalchemy.sql.expression import text
+from sqlalchemy.sql.expression import desc, text
 from titlecase import titlecase
 
 from tildes.enums import ContentMetadataFields, TopicContentType, TopicType
@@ -86,23 +86,14 @@ class Topic(DatabaseModel):
         Integer, ForeignKey("topic_schedule.schedule_id"), index=True
     )
     created_time: datetime = Column(
-        TIMESTAMP(timezone=True),
-        nullable=False,
-        index=True,
-        server_default=text("NOW()"),
+        TIMESTAMP(timezone=True), nullable=False, server_default=text("NOW()"),
     )
     last_edited_time: Optional[datetime] = Column(TIMESTAMP(timezone=True))
     last_activity_time: datetime = Column(
-        TIMESTAMP(timezone=True),
-        nullable=False,
-        index=True,
-        server_default=text("NOW()"),
+        TIMESTAMP(timezone=True), nullable=False, server_default=text("NOW()"),
     )
     last_interesting_activity_time: datetime = Column(
-        TIMESTAMP(timezone=True),
-        nullable=False,
-        index=True,
-        server_default=text("NOW()"),
+        TIMESTAMP(timezone=True), nullable=False, server_default=text("NOW()"),
     )
     is_deleted: bool = Column(
         Boolean, nullable=False, server_default="false", index=True
@@ -125,8 +116,8 @@ class Topic(DatabaseModel):
     content_metadata: Dict[str, Any] = Column(
         MutableDict.as_mutable(JSONB(none_as_null=True))
     )
-    num_comments: int = Column(Integer, nullable=False, server_default="0", index=True)
-    num_votes: int = Column(Integer, nullable=False, server_default="0", index=True)
+    num_comments: int = Column(Integer, nullable=False, server_default="0")
+    num_votes: int = Column(Integer, nullable=False, server_default="0")
     _is_voting_closed: bool = Column(
         "is_voting_closed", Boolean, nullable=False, server_default="false", index=True
     )
@@ -142,6 +133,20 @@ class Topic(DatabaseModel):
     __table_args__ = (
         Index("ix_topics_tags_gist", tags, postgresql_using="gist"),
         Index("ix_topics_search_tsv_gin", "search_tsv", postgresql_using="gin"),
+        # Indexes for keyset pagination
+        Index("ix_topics_created_time_keyset", desc(created_time), desc(topic_id)),
+        Index(
+            "ix_topics_last_activity_time_keyset",
+            desc(last_activity_time),
+            desc(topic_id),
+        ),
+        Index(
+            "ix_topics_last_interesting_activity_time_keyset",
+            desc(last_interesting_activity_time),
+            desc(topic_id),
+        ),
+        Index("ix_topics_num_comments_keyset", desc(num_comments), desc(topic_id)),
+        Index("ix_topics_num_votes_keyset", desc(num_votes), desc(topic_id)),
     )
 
     @hybrid_property  # pylint: disable=used-before-assignment