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How to get the row with the latest value in a TIMESTAMPZ column? Is an index needed? Does an index change the strategy? Would behavior vary by database (I'm using Postgres 9.4)?

My app records data from a data feed. Another process endlessly queries to get the latest freshest entry. Older data may appear on occasion from secondary sources. So the most recently inserted rows are usually, but not necessarily, the freshest data.

I am using this kind of SQL where when_ is a TIMESTAMP WITH TIME ZONE column:

SELECT *
FROM my_table_ 
ORDER BY when_ DESC
LIMIT 1
;

This code works (if no NULL values in data!). But with a possible couple million rows, and a query every 10 seconds, I'm concerned about performance.

Without any index on when_ column, does this statement require a full scan of all rows?

Does adding an index change the performance? Will Postgres automatically scan the index to locate the most recent row, or must I do something to make an index scan happen?

With an index on when_ column, should I change this SQL to use some other approach/strategy of query?

Is there some other way to gather freshly inserted rows? I'm using UUID rather than SERIAL types for my primary key, and may federate data amongst multiple database instances, so that rules out checking for ever-increasing integer numbers.

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  • SELECT *? Do you really need all columns from the row? Or just a small subset? The table definition would be relevant to see width of the row and constraints and types of involved columns. Jul 28, 2015 at 22:34
  • @ErwinBrandstetter The table has a few TIMESTAMP WITH TIME ZONE fields, a couple UUID, and a couple of TEXT fields with 1K to 20K of HTML, CSV, or XML text. I don’t think the other columns are relevant to my question asking how to get the latest row based on a single timestamp column. Primary key is a UUID. There might be a triple-column UNIQUE constraint but not involving the column in question. My real SQL code selects a few of these columns. I am using SELECT * here to keep things simple. Jul 28, 2015 at 23:23
  • I was thinking of a partial, covering index. But if your real SQL code selects a few of these columns. (being 1k - 20k in size) that option is out. A basic table definition is practically always relevant for performance questions like this. Either way, the additional info from your comment should be in the question. Jul 29, 2015 at 0:30
  • @ErwinBrandstetter I am purposely trying to not get specific; I'm not trying to solve a specific performance problem. I thought a general question about how to find latest record in general would be educational for me and many others. Jul 29, 2015 at 0:37
  • 1
    @ErwinBrandstetter I went with a simple index on the when_ column. That server had plenty of RAM and storage space, so no pressing need to conserve. Your function-as-global-constant approach is interesting and educational. Thank you for your excellent Answer. May 18, 2020 at 15:33

1 Answer 1

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Basic answers

Since you select a couple of big columns an index-only scan is probably not a viable option.

This code works (if no NULL values in data!)

While the column isn't defined NOT NULL, add NULLS LAST to the sort order to make it work in any case, even with NULL values. Ideally, use the clause in the corresponding index as well:

SELECT <some big columns>
FROM   my_table_ 
ORDER  BY when_ DESC NULLS LAST
LIMIT  1;

Without any index on when_ column, does this statement require a full scan of all rows?

Yes. Without index, there is no other option left. (Well, there is also table partitioning where an index on key columns(s) is not strictly required, and it could assist with partition pruning. But you would typically have an index on key columns there, too.)

With an index on when_ column, should I change this SQL to use some other approach/strategy of query?

Basically, this is the perfect query. There are options in combination with advanced indexing:


Advanced technique

Assuming a NOT NULL column. Else, add NULLS LAST to index and queries as suggested above.

You have a constant influx of rows with later when_. Assuming the latest _when constantly increases and never (or rarely) decreases (latest rows deleted / updated), you can use a very small partial index.

Basic implementation:

  1. Run your query once to retrieve the latest when_, subtract a safe margin (to be safe against losing the latest rows) and create an IMMUTABLE function based on it. Basically a "fake global constant":

    CREATE OR REPLACE FUNCTION f_when_cutoff()
      RETURNS timestamptz LANGUAGE sql COST 1 IMMUTABLE PARALLEL SAFE AS
    $$SELECT timestamptz '2015-07-25 01:00+02'$$;

    PARALLEL SAFE only in Postgres 9.6 or later.

  2. Create a partial index excluding older rows:

    CREATE INDEX my_table_when_idx ON my_table_ (when_ DESC)
    WHERE when_ > f_when_cutoff();

    With millions of rows, the difference in size can be dramatic. And this only makes sense with a much smaller index. Just half the size or something would not cut it. Index access itself is not slowed much by a bigger index. It's mostly the sheer size of the index, which needs to be read and cached. (And possibly avoiding additional index writes, but hardly in your case.)

  3. Use the function in all related queries. Include the same WHERE condition (even if logically redundant) to convince the query planner the index is applicable. For the simple query:

    SELECT <some big columns>
    FROM   my_table_ 
    WHERE  when_ > f_when_cutoff()
    ORDER  BY when_ DESC
    LIMIT  1;

The size of the index grows with new (later) entries. Recreate the function with a later timestamp and REINDEX from time to time with no or little concurrent access. Only reindex after a relevant number of rows has been added. A couple of thousand entries won't matter much. We are doing this to cut off millions.
The beauty of it: queries don't change.

Implementation with function to update the partial index automatically:

More general advice:

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  • Since we are hard-coding a date-time string in this approach, why create the function f_latest_when? Why not use that hard-coded string directly in the index definition? Jul 29, 2015 at 2:41
  • @BasilBourque: Because you can use f_latest_when() in queries and the index definition alike and don't have to change either after changing f_latest_when(). (But you must reindex.) Jul 29, 2015 at 3:48

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