Re: Proposal: Local indexes for partitioned table

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Cc: Maksim Milyutin <m(dot)milyutin(at)postgrespro(dot)ru>
Subject: Re: Proposal: Local indexes for partitioned table
Date: 2017-10-06 16:37:15
Message-ID: 20171006163715.oulhmymknz3m5mqm@alvherre.pgsql
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I've been thinking about this issue too. I think your patch is not
ambitious enough. Here's my brain dump on the issue, to revive

As you propose, IMO this new feature would use the standard index
creation syntax:
CREATE [UNIQUE] INDEX someindex ON parted_table (a, b);

This command currently throws an error. We'd have it do two things:

1. create catalog rows in pg_class and pg_index for the main table,
indicating the existance of this partitioned index. These would not
point to an actual index (since the main table itself is empty), but
instead to an "abstract" index. This abstract index can be used by
various operations; see below.

2. create one index for each existing partition. These would be
identical to what would happen if you created the index directly on
each partition, except that there is an additional dependency to the
parent's abstract index.

If partitions are themselves partitioned, we would recursively apply the
indexes to the sub-partitions by doing (1) above for the partitioned

Once the index has been created for all existing partitions, the
hierarchy-wide index becomes valid and can be used normally by the
planner/executor. I think we could use the pg_index.indisvalid property
for the abstract index for this.

I propose that an index declared UNIQUE throws an error for now. We can
implement uniqueness (for the case where the indexed columns match the
partitioning key) later, once we sort out all the issues here first. I
think unique indexes would be very useful even with that limitation, but
let's have it as a separate project.

I think using pg_depend as the mechanism to link partition indexes to
parent is a bad idea. How about pg_inherits instead? Seems more

Creating hierachy-wide indexes for existing partitioned tables is likely
to take a long time, so we must include CONCURRENTLY as an option. This
will need some transaction machinery support in order to ensure that
each partition gets its index created at some point in a long chain of
transactions, and that the whole thing is marked valid only at the end.
Also, if the creation is interrupted (because of a crash or a regular
shutdown), it'll be useful to be able to continue rather than being
forced to start from scratch.

When a new partition is added, indexes satisfying the partitioned
table's abstract indexes are created automatically.

During ALTER TABLE ... ATTACH PARTITION, we check that an indexing
satisfying the abstract index exist (and we create a pg_inherit link).
If not, the command is aborted.

REINDEX is easily supported (just reindex each partition's index
individually), but that command is blocking, which is not good. For
concurrent operation for tables not partitioned, a typical pattern to
avoid blocking the table is to suggest creation of an identical index
using CREATE INDEX CONCURRENTLY, then drop the original one. That's not
going to work with these partitioned indexes, which is going to be a
problem. I don't have any great ideas about this part yet.

We need to come up with some way to generate names for each partition

I am going to work on this now.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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