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
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?
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
asked 3 mins ago
dsinecosdsinecos
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I'm trying to understand how Transaction IDs are compared in Postgres
I have a two questions
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?
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
asked 3 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
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?
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
asked 3 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.
I'm trying to understand how Transaction IDs are compared in Postgres
I have a two questions
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?
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
postgresql
asked 3 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 3 mins ago
dsinecosdsinecos
1
New contributor
dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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asked 3 mins ago
dsinecosdsinecos
1
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dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 3 mins ago
dsinecosdsinecos
1
asked 3 mins ago
dsinecosdsinecos
1
1
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dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor
dsinecos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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