Re: Incremental View Maintenance, take 2

On Wed, 28 Jun 2023 00:01:02 +0800
jian he <jian.universality@gmail.com> wrote:

> On Thu, Jun 1, 2023 at 2:47 AM Yugo NAGATA <nagata@sraoss.co.jp> wrote:
> >
> > On Thu, 1 Jun 2023 23:59:09 +0900
> > Yugo NAGATA <nagata@sraoss.co.jp> wrote:
> >
> > > Hello hackers,
> > >
> > > Here's a rebased version of the patch-set adding Incremental View
> > > Maintenance support for PostgreSQL. That was discussed in [1].
> >
> > > [1] https://www.postgresql.org/message-id/flat/20181227215726.4d166b4874f8983a641123f5%40sraoss.co.jp
> >
> > ---------------------------------------------------------------------------------------
> > * Overview
> >
> > Incremental View Maintenance (IVM) is a way to make materialized views
> > up-to-date by computing only incremental changes and applying them on
> > views. IVM is more efficient than REFRESH MATERIALIZED VIEW when
> > only small parts of the view are changed.
> >
> > ** Feature
> >
> > The attached patchset provides a feature that allows materialized views
> > to be updated automatically and incrementally just after a underlying
> > table is modified.
> >
> > You can create an incementally maintainable materialized view (IMMV)
> > by using CREATE INCREMENTAL MATERIALIZED VIEW command.
> >
> > The followings are supported in view definition queries:
> > - SELECT ... FROM ... WHERE ..., joins (inner joins, self-joins)
> > - some built-in aggregate functions (count, sum, avg, min, max)
> > - GROUP BY clause
> > - DISTINCT clause
> >
> > Views can contain multiple tuples with the same content (duplicate tuples).
> >
> > ** Restriction
> >
> > The following are not supported in a view definition:
> > - Outer joins
> > - Aggregates otehr than above, window functions, HAVING
> > - Sub-queries, CTEs
> > - Set operations (UNION, INTERSECT, EXCEPT)
> > - DISTINCT ON, ORDER BY, LIMIT, OFFSET
> >
> > Also, a view definition query cannot contain other views, materialized views,
> > foreign tables, partitioned tables, partitions, VALUES, non-immutable functions,
> > system columns, or expressions that contains aggregates.
> >
> > ---------------------------------------------------------------------------------------
> > * Design
> >
> > An IMMV is maintained using statement-level AFTER triggers.
> > When an IMMV is created, triggers are automatically created on all base
> > tables contained in the view definition query.
> >
> > When a table is modified, changes that occurred in the table are extracted
> > as transition tables in the AFTER triggers. Then, changes that will occur in
> > the view are calculated by a rewritten view dequery in which the modified table
> > is replaced with the transition table.
> >
> > For example, if the view is defined as "SELECT * FROM R, S", and tuples inserted
> > into R are stored in a transiton table dR, the tuples that will be inserted into
> > the view are calculated as the result of "SELECT * FROM dR, S".
> >
> > ** Multiple Tables Modification
> >
> > Multiple tables can be modified in a statement when using triggers, foreign key
> > constraint, or modifying CTEs. When multiple tables are modified, we need
> > the state of tables before the modification.
> >
> > For example, when some tuples, dR and dS, are inserted into R and S respectively,
> > the tuples that will be inserted into the view are calculated by the following
> > two queries:
> >
> >  "SELECT * FROM dR, S_pre"
> >  "SELECT * FROM R, dS"
> >
> > where S_pre is the table before the modification, R is the current state of
> > table, that is, after the modification. This pre-update states of table
> > is calculated by filtering inserted tuples and appending deleted tuples.
> > The subquery that represents pre-update state is generated in get_prestate_rte().
> > Specifically, the insterted tuples are filtered by calling IVM_visible_in_prestate()
> > in WHERE clause. This function checks the visibility of tuples by using
> > the snapshot taken before table modification. The deleted tuples are contained
> > in the old transition table, and this table is appended using UNION ALL.
> >
> > Transition tables for each modification are collected in each AFTER trigger
> > function call. Then, the view maintenance is performed in the last call of
> > the trigger.
> >
> > In the original PostgreSQL, tuplestores of transition tables are freed at the
> > end of each nested query. However, their lifespan needs to be prolonged to
> > the end of the out-most query in order to maintain the view in the last AFTER
> > trigger. For this purpose, SetTransitionTablePreserved is added in trigger.c.
> >
> > ** Duplicate Tulpes
> >
> > When calculating changes that will occur in the view (= delta tables),
> > multiplicity of tuples are calculated by using count(*).
> >
> > When deleting tuples from the view, tuples to be deleted are identified by
> > joining the delta table with the view, and tuples are deleted as many as
> > specified multiplicity by numbered using row_number() function.
> > This is implemented in apply_old_delta().
> >
> > When inserting tuples into the view, each tuple is duplicated to the
> > specified multiplicity using generate_series() function. This is implemented
> > in apply_new_delta().
> >
> > ** DISTINCT clause
> >
> > When DISTINCT is used, the view has a hidden column __ivm_count__ that
> > stores multiplicity for tuples. When tuples are deleted from or inserted into
> > the view, the values of __ivm_count__ column is decreased or increased as many
> > as specified multiplicity. Eventually, when the values becomes zero, the
> > corresponding tuple is deleted from the view.  This is implemented in
> > apply_old_delta_with_count() and apply_new_delta_with_count().
> >
> > ** Aggregates
> >
> > Built-in count sum, avg, min, and max are supported. Whether a given
> > aggregate function can be used or not is checked by using its OID in
> > check_aggregate_supports_ivm().
> >
> > When creating a materialized view containing aggregates, in addition
> > to __ivm_count__, more than one hidden columns for each aggregate are
> > added to the target list. For example, columns for storing sum(x),
> > count(x) are added if we have avg(x). When the view is maintained,
> > aggregated values are updated using these hidden columns, also hidden
> > columns are updated at the same time.
> >
> > The maintenance of aggregated view is performed in
> > apply_old_delta_with_count() and apply_new_delta_with_count(). The SET
> > clauses for updating columns are generated by append_set_clause_*().
> >
> > If the view has min(x) or max(x) and the minimum or maximal value is
> > deleted from a table, we need to update the value to the new min/max
> > recalculated from the tables rather than incremental computation. This
> > is performed in recalc_and_set_values().
> >
> > ---------------------------------------------------------------------------------------
> > * Details of the patch-set (v28)
> >
> > > The patch-set consists of the following eleven patches.
> >
> > In the previous version, the number of patches were nine.
> > In the latest patch-set, the patches are divided more finely
> > aiming to make the review easier.
> >
> > > - 0001: Add a syntax to create Incrementally Maintainable Materialized Views
> >
> > The prposed syntax to create an incrementally maintainable materialized
> > view (IMMV) is;
> >
> >  CREATE INCREMENTAL MATERIALIZED VIEW AS SELECT .....;
> >
> > However, this syntax is tentative, so any suggestions are welcomed.
> >
> > > - 0002: Add relisivm column to pg_class system catalog
> >
> > We add a new field in pg_class to indicate a relation is IMMV.
> > Another alternative is to add a new catalog for managing materialized
> > views including IMMV, but I am not sure if we want this.
> >
> > > - 0003: Allow to prolong life span of transition tables until transaction end
> >
> > This patch fixes the trigger system to allow to prolong lifespan of
> > tuple stores for transition tables until the transaction end. We need
> > this because multiple transition tables have to be preserved until the
> > end of the out-most query when multiple tables are modified by nested
> > triggers. (as explained above in Design - Multiple Tables Modification)
> >
> > If we don't want to change the trigger system in such way, the alternative
> > is to copy the contents of transition tables to other tuplestores, although
> > it needs more time and memory.
> >
> > > - 0004: Add Incremental View Maintenance support to pg_dump
> >
> > This patch enables pg_dump to output IMMV using the new syntax.
> >
> > > - 0005: Add Incremental View Maintenance support to psql
> >
> > This patch implements tab-completion for the new syntax and adds
> > information of IMMV to \d meta-command results.
> >
> > > - 0006: Add Incremental View Maintenance support
> >
> > This patch implements the basic IVM feature.
> > DISTINCT and aggregate are not supported here.
> >
> > When an IMMV is created, the view query is checked, and if any
> > non-supported feature is used, it raises an error. If it is ok,
> > triggers are created on base tables and an unique index is
> > created on the view if possible.
> >
> > In BEFORE trigger, an entry is created for each IMMV and the number
> > of trigger firing is counted. Also, the snapshot just before the
> > table modification is stored.
> >
> > In AFTER triggers, each transition tables are preserved. The number
> > of trigger firing is counted also here, and when the firing number of
> > BEFORE and AFTER trigger reach the same, it is deemed the final AFTER
> > trigger call.
> >
> > In the final AFTER trigger, the IMMV is maintained. Rewritten view
> > query is executed to generate delta tables, and deltas are applied
> > to the view. If multiple tables are modified simultaneously, this
> > process is iterated for each modified table. Tables before processed
> > are represented in "pre-update-state", processed tables are
> > "post-update-state" in the rewritten query.
> >
> > > - 0007: Add DISTINCT support for IVM
> >
> > This patch adds DISTINCT clause support.
> >
> > When an IMMV including DISTINCT is created, a hidden column
> > "__ivm_count__" is added to the target list. This column has the
> > number of duplicity of the same tuples. The duplicity is calculated
> > by adding "count(*)" and GROUP BY to the view query.
> >
> > When an IMMV is maintained, the duplicity in __ivm_count__ is updated,
> > and a tuples whose duplicity becomes zero can be deleted from the view.
> > This logic is implemented by SQL in apply_old_delta_with_count and
> > apply_new_delta_with_count.
> >
> > Columns starting with "__ivm_" are deemed hidden columns that doesn't
> > appear when a view is accessed by "SELECT * FROM ....".  This is
> > implemented by fixing parse_relation.c.
> >
> > > - 0008: Add aggregates support in IVM
> >
> > This patch provides codes for aggregates support, specifically
> > for builtin count, sum, and avg.
> >
> > When an IMMV containing an aggregate is created, it is checked if this
> > aggregate function is supported, and if it is ok, some hidden columns
> > are added to the target list.
> >
> > When the IMMV is maintained, the aggregated value is updated as well as
> > related hidden columns. The way of update depends the type of aggregate
> > functions, and SET clause string is generated for each aggregate.
> >
> > > - 0009: Add support for min/max aggregates for IVM
> >
> > This patch adds min/max aggregates support.
> >
> > This is separated from #0008 because min/max needs more complicated
> > work than count, sum, and avg.
> >
> > If the view has min(x) or max(x) and the minimum or maximal value is
> > deleted from a table, we need to update the value to the new min/max
> > recalculated from the tables rather than incremental computation.
> > This is performed in recalc_and_set_values().
> >
> > TIDs and keys of tuples that need re-calculation are returned as a
> > result of the query that deleted min/max values from the view using
> > RETURNING clause. The plan to recalculate and set the new min/max value
> > are stored and reused.
> >
> > > - 0010: regression tests
> >
> > This patch provides regression tests for IVM.
> >
> > > - 0011: documentation
> >
> > This patch provides documantation for IVM.
> >
> > ---------------------------------------------------------------------------------------
> > * Changes from the Previous Version (v27)
> >
> > - Allow TRUNCATE on base tables
> >
> > When a base table is truncated, the view content will be empty if the
> > view definition query does not contain an aggregate without a GROUP clause.
> > Therefore, such views can be truncated.
> >
> > Aggregate views without a GROUP clause always have one row. Therefore,
> > if a base table is truncated, the view will not be empty and will contain
> > a row with NULL value (or 0 for count()). So, in this case, we refresh the
> > view instead of truncating it.
> >
> > - Fix bugs reported by huyajun [1]
> >
> > [1] https://www.postgresql.org/message-id/tencent_FCAF11BCA5003FD16BDDFDDA5D6A19587809%40qq.com
> >
> > ---------------------------------------------------------------------------------------
> > * Discussion
> >
> > ** Aggregate support
> >
> > There were a few suggestions that general aggregate functions should be
> > supported [2][3], which may be possible by extending pg_aggregate catalog.
> > However, we decided to leave supporting general aggregates to the future work [4]
> > because it would need substantial works and make the patch more complex and
> > bigger.
> >
> > There has been no opposite opinion on this. However, if we need more discussion
> > on the design of aggregate support, we can omit aggregate support for the first
> > release of IVM.
> >
> > [2] https://www.postgresql.org/message-id/20191128140333.GA25947%40alvherre.pgsql
> > [3] https://www.postgresql.org/message-id/CAM-w4HOvDrL4ou6m%3D592zUiKGVzTcOpNj-d_cJqzL00fdsS5kg%40mail.gmail.com
> > [4] https://www.postgresql.org/message-id/20201016193034.9a4c44c79fc1eca7babe093e%40sraoss.co.jp
> >
> > ** Hidden columns
> >
> > In order to support DISTINCT or aggregates, our implementation uses hidden columns.
> >
> > Columns starting with "__ivm_" are hidden columns that doesn't appear when a
> > view is accessed by "SELECT * FROM ....". For this aim, parse_relation.c is
> > fixed. There was a proposal to enable hidden columns by adding a new flag to
> > pg_attribute [5], but this thread is no longer active, so we decided to check
> > the hidden column by its name [6].
> >
> > [5]
https://www.postgresql.org/message-id/flat/CAEepm%3D3ZHh%3Dp0nEEnVbs1Dig_UShPzHUcMNAqvDQUgYgcDo-pA%40mail.gmail.com
> > [6] https://www.postgresql.org/message-id/20201016193034.9a4c44c79fc1eca7babe093e%40sraoss.co.jp
> >
> > ** Concurrent Transactions
> >
> > When the view definition has more than one table, we acquire an exclusive
> > lock before the view maintenance in order to avoid inconsistent results.
> > This behavior was explained in [7]. The lock was improved to use weaker lock
> > when the view has only one table based on a suggestion from Konstantin Knizhnik [8].
> > However, due to the implementation that uses ctid for identifying target tuples,
> > we still have to use an exclusive lock for DELETE and UPDATE.
> >
> > [7] https://www.postgresql.org/message-id/20200909092752.c91758a1bec3479668e82643%40sraoss.co.jp
> > [8] https://www.postgresql.org/message-id/5663f5f0-48af-686c-bf3c-62d279567e2a%40postgrespro.ru
> >
> > ** Automatic Index Creation
> >
> > When a view is created, a unique index is automatically created if
> > possible, that is, if the view definition query has a GROUP BY or
> > DISTINCT, or if the view contains all primary key attributes of
> > its base tables in the target list. It is necessary for efficient
> > view maintenance. This feature is based on a suggestion from
> > Konstantin Knizhnik [9].
> >
> > [9] https://www.postgresql.org/message-id/89729da8-9042-7ea0-95af-e415df6da14d%40postgrespro.ru
> >
> >
> > ** Trigger and Transition Tables
> >
> > We implemented IVM based on triggers. This is because we want to use
> > transition tables to extract changes on base tables. Also, there are
> > other constraint that are using triggers in its implementation, like
> > foreign references. However, if we can use transition table like feature
> > without relying triggers, we don't have to insist to use triggers and we
> > might implement IVM in the executor directly as similar as declarative
> > partitioning.
> >
> > ** Feature to be Supported in the First Release
> >
> > The current patch-set supports DISTINCT and aggregates for built-in count,
> > sum, avg, min and max. Do we need all these feature for the first IVM release?
> > Supporting DISTINCT and aggregates needs discussion on hidden columns, and
> > for supporting min/max we need to discuss on  re-calculation method. Before
> > handling such relatively advanced feature, maybe, should we focus to design
> > and implement of the basic feature of IVM?
> >
> >
> > Any suggestion and discussion are welcomed!
> >
> > Regards,
> > Yugo Nagata
> >
> > --
> > Yugo NAGATA <nagata@sraoss.co.jp>
> >
> >
> 
> 
> > The followings are supported in view definition queries:
> > - SELECT ... FROM ... WHERE ..., joins (inner joins, self-joins)
> 
> 
> > Also, a view definition query cannot contain other views, materialized views,
> > foreign tables, partitioned tables, partitions, VALUES, non-immutable functions,
> > system columns, or expressions that contains aggregates.
> 
> Does this also apply to tableoid?  but tableoid is a constant, so it
> should be fine?
> can following two queries apply to this feature.
> select tableoid, unique1 from tenk1;

Currently, this is not allowed because tableoid is a system column.
As you say, tableoid is a constant, so we can allow. Should we do this?

> select 1 as constant, unique1 from tenk1;

This is allowed, of course.

> I didn't apply the patch.(will do later, for someone to test, it would
> be a better idea to dump a whole file separately....).

Thank you! I'm looking forward to your feedback.
(I didn't attach a whole patch separately because I wouldn't like
cfbot to be unhappy...)

Regards,
Yugo Nagata

-- 
Yugo NAGATA <nagata@sraoss.co.jp>