PostgreSQL – WHERE Clause Converting INT to DATETIME

I must first compliment you on your courage to do something like this with an Access DB, which from my experience is very difficult to do anything SQL-like. Anyways, on to the review.

First join

Your IIF field selections might benefit from using a Switch statement instead. It seems to be sometimes the case, especially with things SQL, that a SWITCH (more commonly known as CASE in typical SQL) is quite fast when just making simple comparisons in the body of a SELECT. The syntax in your case would be almost identical, although a switch can be expanded to cover a large chunk of comparisons in one field. Something to consider.

  SWITCH (
    expr1, val1,
    expr2, val2,
    val3        -- default value or "else"
  )

A switch can also help readability, in larger statements. In context:

  MAX(SWITCH(B.XTStamp <= A.RecTStamp,B.XTStamp,Null)) as BeforeXTStamp,
  --alternatively MAX(-(B.XTStamp<=A.RecTStamp)*B.XTStamp) as BeforeXTStamp,
  MIN(SWITCH(B.XTStamp>A.RecTStamp,B.XTStamp,Null)) as AfterXTStamp

As for the join itself, I think (A.RecTStamp<>B.XTStamp OR A.RecTStamp=B.XTStamp) is about as good as you're going to get, given what you are trying to do. It's not that fast, but I wouldn't expect it to be either.

Second join

You said this is slower. It's also less readable from a code standpoint. Given equally satisfactory result sets between 1 and 2, I'd say go for 1. At least it's obvious what you are trying to do that way. Subqueries are often not very fast (though often unavoidable) especially in this case you are throwing in an extra join in each, which must certainly complicate the execution plan.

One remark, I saw that you used old ANSI-89 join syntax. It's best to avoid that, the performance will be same or better with the more modern join syntax, and they are less ambiguous or easier to read, harder to make mistakes.

FROM (FirstTable AS A INNER JOIN 
  (select top 1 B1.XTStamp, A1.RecTStamp 
   from SecondTable as B1
   inner join FirstTable as A1
     on B1.XTStamp <= A1.RecTStamp
   order by B1.XTStamp DESC) AS AbyB1 --MAX (time points before)

Naming things

I think the way your things are named is unhelpful at best, and cryptic at worst. A, B, A1, B1 etc. as table aliases I think could be better. Also, I think the field names are not very good, but I realize you may not have control over this. I will just quickly quote The Codeless Code on the topic of naming things, and leave it at that...

“Invective!” answered the priestess. “Verb your expletive nouns!”

"Next steps" query

I couldn't make much sense of it how it was written, I had to take it to a text editor and do some style changes to make it more readable. I know Access' SQL editor is beyond clunky, so I usually write my queries in a good editor like Notepad++ or Sublime Text. Some of the stylistic changes I applied to make it more readable:

• 4 spaces indent instead of 2 spaces
• Spaces around mathematical and comparison operators
• More natural placing of braces and indentation (I went with Java-style braces, but could also be C-style, at your preference)

So as it turns out, this is a very complicated query indeed. To make sense of it, I have to start from the innermost query, your ID data set, which I understand is the same as your First Join. It returns the IDs and timestamps of the devices where the before/after timestamps are the closest, within the subset of devices you are interested in. So instead of ID why not call it ClosestTimestampID.

Your Det join is used only once:

The rest of the time, it only joins the values you already have from ClosestTimestampID. So instead we should be able to just do this:

    ) AS ClosestTimestampID
    INNER JOIN SecondTable AS TL1 
        ON ClosestTimestampID.BeforeXTStamp = TL1.XTStamp) 
    INNER JOIN SecondTable AS TL2 
        ON ClosestTimestampID.AfterXTStamp = TL2.XTStamp
    WHERE ClosestTimestampID.XmitID IN (<limited subset S>)

Maybe not be a huge performance gain, but anything we can do to help the poor Jet DB optimizer will help!

I can't shake the feeling that the calculations/algorithm for BeforeWeight and AfterWeight which you use to interpolate could be done better, but unfortunately I'm not very good with those.

One suggestion to avoid crashing (although it's not ideal depending on your application) would be to break out your nested subqueries into tables of their own and update those when needed. I'm not sure how often you need your source data to be refreshed, but if it is not too often you might think of writing some VBA code to schedule an update of the tables and derived tables, and just leave your outermost query to pull from those tables instead of the original source. Just a thought, like I said not ideal but given the tool you may not have a choice.

Everything together:

SELECT
    InGPS.XmitID,
    StrDateIso8601Msec(InGPS.RecTStamp) AS RecTStamp_ms,
       -- StrDateIso8601MSec is a VBA function returning a TEXT string in yyyy-mm-dd hh:nn:ss.lll format
    InGPS.ReceivID,
    RD.Receiver_Location_Description,
    RD.Lat AS Receiver_Lat,
    RD.Lon AS Receiver_Lon,
    InGPS.Before_Lat * InGPS.BeforeWeight + InGPS.After_Lat * InGPS.AfterWeight AS Xmit_Lat,
    InGPS.Before_Lon * InGPS.BeforeWeight + InGPS.After_Lon * InGPS.AfterWeight AS Xmit_Lon,
    InGPS.RecTStamp AS RecTStamp_basic
FROM (
    SELECT 
        ClosestTimestampID.RecTStamp,
        ClosestTimestampID.XmitID,
        ClosestTimestampID.ReceivID,
        ClosestTimestampID.BeforeXTStamp, 
        TL1.Latitude AS Before_Lat, 
        TL1.Longitude AS Before_Lon,
        (1 - ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) 
            / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp))) AS BeforeWeight,
        ClosestTimestampID.AfterXTStamp, 
        TL2.Latitude AS After_Lat, 
        TL2.Longitude AS After_Lon,
        (     (ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) 
            / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp)) AS AfterWeight
        FROM (((
            SELECT 
                A.RecTStamp, 
                A.ReceivID, 
                A.XmitID,
                MAX(SWITCH(B.XTStamp <= A.RecTStamp, B.XTStamp, Null)) AS BeforeXTStamp,
                MIN(SWITCH(B.XTStamp > A.RecTStamp, B.XTStamp, Null)) AS AfterXTStamp
            FROM FirstTable AS A
            INNER JOIN SecondTable AS B 
                ON (A.RecTStamp <> B.XTStamp OR A.RecTStamp = B.XTStamp)
            WHERE A.XmitID IN (<limited subset S>)
            GROUP BY A.RecTStamp, ReceivID, XmitID
        ) AS ClosestTimestampID
        INNER JOIN FirstTable AS Det 
            ON (Det.XmitID = ClosestTimestampID.XmitID) 
            AND (Det.ReceivID = ClosestTimestampID.ReceivID) 
            AND (Det.RecTStamp = ClosestTimestampID.RecTStamp)) 
        INNER JOIN SecondTable AS TL1 
            ON ClosestTimestampID.BeforeXTStamp = TL1.XTStamp) 
        INNER JOIN SecondTable AS TL2 
            ON ClosestTimestampID.AfterXTStamp = TL2.XTStamp
        WHERE Det.XmitID IN (<limited subset S>)
    ) AS InGPS
INNER JOIN ReceiverDetails AS RD 
    ON (InGPS.ReceivID = RD.ReceivID) 
    AND (InGPS.RecTStamp BETWEEN <valid parameters from another table>)
ORDER BY StrDateIso8601Msec(InGPS.RecTStamp), InGPS.ReceivID;

Your subquery is uncorrelated and returns all rows from the join between native_name_aliases and dpoint, where only a single row would be allowed. I guess this is what you are after:

UPDATE dpoint d
SET    native_name = nna.native_name
FROM   native_name_aliases nna
WHERE  d.building_id = 42
AND    d.dpoint_id = nna.dpoint_id
AND    d.native_name IS DISTINCT FROM nna.native_name;

You did not provide table definitions and Postgres version, so I can't tell for sure.

The last line is to prevent expensive "empty" updates. Details:

• How do I (or can I) SELECT DISTINCT on multiple columns?