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How are Transaction IDs compared in Postgres?

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I'm trying to understand how Transaction IDs are compared in Postgres

I have a two questions

1. Transaction IDs (XIDs) are represented in 32 bits. Therefore there can be a maximum of 4,294,967,296 XIDs. Why is a wraparound possible at two billion XIDs?




2. How are XIDs compared?

Consider that there are only 3 bits to represent XIDs allowing the following values for XID - 0,1,2,3,4,5,6,7.

Before a wraparound XIDs can be simply compared x < y.

However after a wraparound, say the last XID was 7 and the next XID would be 1. How will these be compared to establish that XID 1 is greater than XID 7 (to allow visibility of the row with xmin as 7)

In my research I came across the following but I'm at a loss how the comparison works

From the Postgres docs

Normal XIDs are compared using modulo-2^32 arithmetic. This means that for every normal XID, there are two billion XIDs that are “older” and two billion that are “newer”; another way to say it is that the normal XID space is circular with no endpoint

From a post by Tom Lane

Ordered comparisons on XIDs are no longer simply "x < y", but need to be expressed as a macro. We consider x < y if (y - x) % WRAPLIMIT < WRAPLIMIT/2.
This comparison will work as long as the range of interesting XIDs
never exceeds WRAPLIMIT/2

postgresql

share

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0

I'm trying to understand how Transaction IDs are compared in Postgres

I have a two questions

1. Transaction IDs (XIDs) are represented in 32 bits. Therefore there can be a maximum of 4,294,967,296 XIDs. Why is a wraparound possible at two billion XIDs?




2. How are XIDs compared?

Consider that there are only 3 bits to represent XIDs allowing the following values for XID - 0,1,2,3,4,5,6,7.

Before a wraparound XIDs can be simply compared x < y.

However after a wraparound, say the last XID was 7 and the next XID would be 1. How will these be compared to establish that XID 1 is greater than XID 7 (to allow visibility of the row with xmin as 7)

In my research I came across the following but I'm at a loss how the comparison works

From the Postgres docs

Normal XIDs are compared using modulo-2^32 arithmetic. This means that for every normal XID, there are two billion XIDs that are “older” and two billion that are “newer”; another way to say it is that the normal XID space is circular with no endpoint

From a post by Tom Lane

Ordered comparisons on XIDs are no longer simply "x < y", but need to be expressed as a macro. We consider x < y if (y - x) % WRAPLIMIT < WRAPLIMIT/2.
This comparison will work as long as the range of interesting XIDs
never exceeds WRAPLIMIT/2

postgresql

share

asked 2 mins ago

dsinecosdsinecos

1

New contributor

dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

I'm trying to understand how Transaction IDs are compared in Postgres

I have a two questions

1. Transaction IDs (XIDs) are represented in 32 bits. Therefore there can be a maximum of 4,294,967,296 XIDs. Why is a wraparound possible at two billion XIDs?




2. How are XIDs compared?

Consider that there are only 3 bits to represent XIDs allowing the following values for XID - 0,1,2,3,4,5,6,7.

Before a wraparound XIDs can be simply compared x < y.

However after a wraparound, say the last XID was 7 and the next XID would be 1. How will these be compared to establish that XID 1 is greater than XID 7 (to allow visibility of the row with xmin as 7)

In my research I came across the following but I'm at a loss how the comparison works

From the Postgres docs

Normal XIDs are compared using modulo-2^32 arithmetic. This means that for every normal XID, there are two billion XIDs that are “older” and two billion that are “newer”; another way to say it is that the normal XID space is circular with no endpoint

From a post by Tom Lane

Ordered comparisons on XIDs are no longer simply "x < y", but need to be expressed as a macro. We consider x < y if (y - x) % WRAPLIMIT < WRAPLIMIT/2.
This comparison will work as long as the range of interesting XIDs
never exceeds WRAPLIMIT/2

postgresql

share

asked 2 mins ago

dsinecosdsinecos

1

New contributor

dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

asked 2 mins ago

dsinecosdsinecos

1

New contributor

dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

asked 2 mins ago

dsinecosdsinecos

1

New contributor

dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 mins ago

dsinecosdsinecos

1

New contributor

dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 mins ago

dsinecosdsinecos

1